4.0/4.0 vs. 4.5/3.5 is a different game. Maybe 4.5s shouldn't play 8.0 mixed, but it does eliminate playing opportunities for 4.5 men. I personally wouldn't care that much, but it would be hard for me to play Mixed if 9.0 was my only option. I practically had to beg to be on a 9.0 team, and we weren't very good, I only played two matches, both losses, and both matches I had a 4.0 partner. There just aren't very many 4.5 women to go around for the 4.5 guys.
But playing 8.0 mixed does allow me to play with friends that I otherwise would not be able to play with, which is nice. The problem is that there is such a huge range of 4.5s out there. I think most people wouldn't have a problem playing against me, though I usually win at 8.0 mixed, but I don't really blow anyone out, my game is very balanced and defense-oriented and I am not physically imposing. On the other hand, there is a guy on my 8.0 mixed team who is about 6'6", with a huge lefty serve. When he hits his kick serve, it often bounces over the woman's head and she has no shot at it, even good 4.0 women have no shot at returning the serve. But yet the rest of his game can be error-prone and he is beatable by the right players.
But all of this is kind of a moot point, because the USTA is not going to take less money and restrict the number of leagues that those 4.5s can play in.