2 main reasons I do not use all you can eat subscriptions:
1) I like to KEEP my books (in Calibre). I never know when/if I will re-read.
2) I am picky. I don't always read the same quantity or story type every month.
0) I am cheap

I looked at my years
buying history, I did not buy $120 in books any one store. Baen got the most last year