This rule is awful for mixed. You can have players of different genders of different levels make nationals, but none of them together. Now, you can not play with your partner of many years.
On one of my 4.5 teams we had two guys who were 4.0 play in as subs because we couldn't get anyone else to play. They didn't qualify for nationals and obviously aren't going, but are technically on our roster. Now, they count as players who went to nationals and are being punished for helping out our team as a sub. Had they known this rule was going into place, they wouldn't have played and now have sacrificed their own play for 2013-2014. USTA really didn't think this through. I can understand not being able to play at your own level with more than 3 players who went to nationals, but on a mixed team or a team played out of your own level, or for those who didn't qualify for nationals, it doesn't make sense. I would go as far to say that if you don't physically play at nationals, you shouldn't be part of this rule.
I totally 100% agree with this. I will say, though, that I also got an email from Middle States that says the move up/split up rule does not apply at all for Mixed 40+ because it's not a national program. So, mixed has either (1) the harshest restrictions of all classes in 18s because you have to track nationals players in tons of leagues and levels at once or (2) no restriction at all in 40+.