I keep the small ones (2" or more) to use as "test" pieces. Whether I'm testing thread and tension, thread for the "look" on the quilt (especially variagated thread), or practicing new techniques. Also keep small ones in the event that you need them for a trapunto design.
I have sewn batting together, if I had two large pieces and the two together was enough for a quilt. I've just zig-zagged the edges.