cindy -- in an ideal world, i'm not really a fan of a 4.5 guy playing with a 3.5 lady. because it is too large a gap in skill level. and this forces people to play a type of tennis that you don't normally play.
however -- until the USTA decides to have a gender neutral NTRP rating system where a 4.5 guy and a 4.5 woman are equivalent . . . OR a rule that says the woman must be equal or higher rated than the guy in mixed -- it is what it is.
btw - it's also too simplistic to say that the best teams have the most 4.5 guys. that team in 6th place should by your formula be in the top 3.
the REAL formula for success is the same for all usta leagues - mens, womens, mixed, combo, trilevel, etc. the best teams are the ones who have the most players who are at the very top of their respective rating. as in top level 3.0 or 3.5 or 4.0 or 4.5 players. period.