Tomorrow I will post my pictures of the dresses and you will be able to see the difference. If you are sewing for utilitarian purposes (play clothes, dress up clothes, house clothes ) then old tees are worth cutting up. However, if the pattern involves detailed work, or new fabrics to embellish, it may not be worth the time. Old tees look old, and it is a pity to put in a lot of extra effort and creativity when the result looks "old or used".
If you are merely attaching a tee top with a simple attachment seam to one bottom piece of fabric it may be nice enough, but detailed work may be wasting time best given to a "new" item.