The other piece to pressing your seams to one side has to do withthe actual quilting. If you press your seams open and then decide the stitch in the ditch, you will not be sewing through any actual fabric on your blocks, only the threads. this could cause issues as your quilt is used.
Like the other ladies said, a 1/2in seam may not be a big deal on some blocks, but on a block where you may have 6 seams intersecting, that is going to create a lot of bulk.
The other piece is, that you would be using a lot more fabric overall on a quilt. Most patterns go one the assumption that you are using the 1/4in seam and direct you to buy fabric accordingly. If you are going to use 1/2in seams, you will need to figure in that extra fabric if you are using a pattern. Think how quickly a 1/16 of an inch adds up over just one block, then multiply that!