The answer depends upon several things:
- the features of the dress you are altering, such as, sleeveless vs sleeved, types of sleeves if present, gathered vs fitted waist, presence of side seam fasteners or pockets, etc...
- the fabric the dress is made of
- how much you are altering the dress
- how persnickety you yourself are about things like seam finishes and drape of the finished product (as noted by @SueC and @Stefan in the comments above)
In the first youtube video you site, the front and back pieces of the bodice and skirt are each one continuous piece of fabric, so reducing the circumferences of the body and sleeves a small amount by simply stitching along the side seams and underarm seams (what my Mom calls “taking in the seams”) would probably work “well enough”. However, Kurtis are made of woven fabric, and the sleeves are quite fitted, making alterations of anything more than a small amount potentially unforgiving; one would need to proceed with caution, the explanation of which will take up most of the rest of this answer...
So yes, this scenario could possibly apply to your alteration of a dress with a gathered skirt, from the standpoint of re-fitting the basically-tubular shapes of bodice and skirt in a relatively straightforward manner. That is, and here’s the tricky part, “unless” your dress has sleeves, is made of a woven fabric, and / or there will be a large amount of fabric left in the new seam allowance, in which cases the ability of the shoulder seam to lay nicely and articulate normally could be hindered.
This is because the armscye is one of the most complex places to alter on a torso garment (dress, shirt, jacket...), as both the size and shape of the wearer, and motions of the arm and shoulder in actions such as reaching forward or overhead, are involved. I would consider this an advanced skill in the alteration realm.
Certainly this depends on the dress you are altering. If it is sleeveless, and / or made of a knit fabric, you will possibly have less of an issue altering at the armscye. But even on a sleeveless dress, it is easy for an armscye to become too small or too gaping during the alteration process.
The second and third videos you site demonstrate cutting the too-large dresses apart first at the waist, then at the side seams, then reassembling them to a smaller size. In the second video, the dress has to be trimmed down extensively at the sides because the original dress had pleats, as well as being 10 sizes too large. A sheath dress such as her finished product is meant to fit just right, conforming to the figure, so cutting away a great amount of material is necessary, which obviously leaves raw edges for seaming... She wisely uses a pencil skirt that she likes the fit of as a pattern for cutting away the sides of the skirt, which also allows her to confirm that the amount she takes off of the side seams of the bodice is correct, since the two circumferences apparently match.
The lady who alters down a thrift shop dress basically cuts all the pieces apart, which is somewhat like starting from where you would when cutting out a pattern on new fabric. The difference being that from there, she needs a method to know how much to take in, and where, which she doesn’t explain. She had to either use her own measurements, taking positive ease into consideration, or she used a dress she likes the fit of as a pattern, or she used an actual sewing pattern. That dress is of knit fabric, so I assume she is using a serger or a stretchy stitch on a regular sewing machine; both methods would lend a lot of forgiveness to the fit, if the reduction method she uses is less than perfect. Also note, it is a sleeveless dress, which obviates most of the potential complications of fitting the articulation of the shoulder joint.
So, the fourth video seems to express your question most directly. Wouldn’t she have saved time and effort by merely “taking in the seams”? Possibly yes, but in my experience, I don’t think so. Notice that at 3:09, she trims the armscye. She feels this is necessary as evidenced by her statement there that she determined how much to trim “from the shoulder seam” (while the cut she makes actually affects both the armscye and shoulder seams) by measuring “across from one shoulder to another”. She did shorten the length of the shoulder seam, which makes it a smaller size, BUT although she states that this action has not changed the size of the armhole, it actually has: it is now larger and deeper, which counterintuitively makes it less articulating. This website, “Parisian Gentleman”, explains this phenomenon well:
...if the armscye fits close to the armpit, then movement of the arms will not be restricted and the front of the jacket will not move when your arms move. Conversely, an armscye positioned too low, leaving too much space between the armpit and the bottom of the armscye will cause a small sartorial catastrophe which includes discomfort, strain on the mid part of the jacket when the arms move and riding up of the back of the jacket when seated at a table with arms forward.
I have made this mistake myself, and believe me, it is very uncomfortable, you feel like your upper arms are tied to your torso!
So again, could you take your dress and merely “take in the seams”? Very possibly, depending upon these many factors, chief among which is that pesky armscye. The gal in the last video certainly could have made all of her changes, except the alteration of the armscye, by merely “taking in the seams”.
One simple strategy to test your theory that cutting apart the seams doesn’t always seem necessary, would be to baste the seams you intend to take in, about 1/16th inch outside of (that is, closer to the edge from) where you want the seams to go. (This way the basting can be easily removed later, as it will not be sewn over, OR you could perhaps choose to leave it in...) When you try the basting-altered garment on, move your arms forward and upward, in reaching motions, to test the armscye fit. One caveat: the seam allowance in the armscye area might be pulling things tightly even though the seam is placed correctly, in which case some very strategic and careful notch placement would be in order, which might also require you to finish that section of the seam...
The last “depends upon” factor to mention is, your personal standards and preferences for finishing seams. In some situations this is strictly personal preference, but sometimes the way a seam is finished does affect the outcome of the garment so strongly that the finished garment is either the best thing you ever wore, or it is never worn again after trying it on. This is because things like waving or puckers in the seam allowance, or, pulling or twisting of the garment around constricted areas like the waist or arms, can occur to a surprisingly severe degree, simply due to an improperly finished seam. It has to do with the behavior of fabric that is stitched.
Understanding this personality quirk, which is different among each of the fabrics you will sew with, comes with study and experience. The best place to start is what you are doing: follow your instincts and try things. It sounds like you are experimenting with some interesting seam finishes (although I have never heard of a “weft seam”, except with hand-woven fabrics?) There are many ways to any given result in sewing, and no single right way to do anything. Go forth and try what your mind is suggesting, and see how it works out!
Since I have talked about a couple of important and specific sewing skill sets, here are some links for further study:
How Seam Finishes May Affect The Finished Garment:
SEAM FINISHES - A seam finish is any technique that is used to make a seam neat, to prevent the seam allowance edges from raveling and/or fraying, to prevent seam allowances from rolling, to prevent stretching and rippling in some seam allowances, and to give the inside of the garment a more pleasing appearance. Examples of seam finishes are pinked, overcast, Hong Kong, turned and stitched, and serged.
- The seam finish is appropriate to the garment fabric, the garment design, the intended use of the garment, its quality, and garment care.
- The finish is applied securely so that it remains in place during normal wear and care.
- The finish does not add bulk to the seam.
- The selected finish prevents the fabric from raveling, rolling, or
stretching and contributes to the overall neatness of the garment.
- The finishes’ binding or thread color is appropriate to the fashion fabric, unless used for decorative purposes.