I swap, but only once in the last year. This is because I MUST reduce my stash and it is rare for my stash to have the requirements for a swap. My own fault really - lots of novelties. The second would be a swap that comes up at a time when I really can't participate because of other obligations or because I simply am not doing well (I am disabled). Sometimes I just get shut out of a swap. I want to participate, but need to find fabric and do the block to see if I can do a reasonable job. Then I go to sign up and it's already closed.
I totally understand the issue of not completing the tops. That's why I always make enough blocks to finish the quilt and select my swap blocks from those. Otherwise, it tends to languish if I have to make more after the swap arrives.