... Because if you did it would remove the usefulness of the spring FA draft.

I like the fact that you only get one shot per year at FA's in TRAD Leagues.

(1) It makes you think about what you want to do in those later rounds

(2) If we had monthly drafts then it would drain the talent available the following spring. This means noone would do trades with FA picks which means less pennant run trades can get done.