It's true that the size of the seam will not matter as long as you are making a quilt out of all of the same block pattern, or all of the same block pattern alternating with plain blocks.
Problems arise quickly, however, when you want to make a quilt that uses multiple types of pieced block. The most obvious example would be a sampler quilt. Even if your seams are a consistent size, every block may end up a different size because their patterns are different.
I'm trying to think of two block patterns that would illustrate this clearly, but my mind is blank. I found this article that tries to illustrate the problem:
http://www.scrapquilts.com/accurate_seams.html
There isn't so much of a problem when you are dealing with squares. The problems start to become more obvious when making blocks with star points.
I'm thinking there would be problems even with a feathered star if you cut all the pieces out first. If the star strips are not sewn with the correct 1/4" seam, they will not fit the already-cut center squares. If you know this in advance, you can make the strips first and then cut the squares to fit the star points. However, again you may have the problem of the feathered star block not being the same size as another block featured in the quilt.
Those of you who have not had a problem probably are using scant 1/4" seams without realizing it. Since all block patterns are sized assuming an accurate finished 1/4" seam, all block patterns will come out the same size for you if you happen to be using a scant 1/4" already.