As a 4.5 guy, I have played with and against many advanced female players including ex college players. From 4.0 to 5.5. Men and women just play different esp as they get better and better. At the 4.5 level, I play many types of advanced men players including guys with huge and/or spinny and/or tricky serves. Men are also taller and quicker at the net. Much better at net play, covering the court, and net positioning. Females who can and do play like this are very rare. I can probably count on one hand the number of female serve and volleyers. All the 5.0 ladies I know are great and consistent baseliners but there is not significant play style differences amongst them.
If you combined the ratings, there essentially would not be any 4.5 females. (Please note, I am not saying all men are better then women, just that women who played at the same level as a 4.5 man or better are very very very rare).
I find the current mixed system to be just fine. Any proposed system with ratings, they all have their own inherent pros and cons. An actual combined rating system would distribute the ratings 2 far across a wide gap.