I may be wrong because the sample size here is small, but I dont think many 5.0's play USTA league tennis. This "rule" sounds like an effort to get more people into adult leagues which I think is a good thing.
I know a few college players who have not signed up for USTA because trying to find a 5.0/9.0 team is nearly impossible here. I dont think there are any 5.0 teams, and the 9.0 combos are very small, about the size of 2.5, and maybe 3.0 now days.
A 5.0 self-rate is not something any wants to be when they're looking for a team simply because they're either a self-rate (risk) or they're limited to 9.0 doubles since 5.0 adult is practically non-existent.
I can totally see this as a business move for USTA to get 5.0's active in adult league since they can now play 4.5 and not be limited to 9.0