If your users are rotating avatars such that you're storing 50 new avatar versions a day, then all the sudden you're consuming an additional 6GB of disk space this month, even though volume on your forum hasn't really changed.
... well ok? But. Since they can already store unlimited images, what's the diff?
Not saying it's the end of the world to do it that way, but I've definitely seen multiple servers wind up with critical outages because the lack of a proper tombstoning strategy for old, irrelevant data (log files, in that case, but the same thing applies here to old avatars) led to an out of disk space problem and everything fell over.
I really doubt the "old" avatars would even be a drop in the bucket compared to the number of times I've uploaded the "cool story bro" comic panel.