Multi battle gyms sound cool because of the idea of Tate and Liza, but I agree with them not being the best idea because of time zones and setting them up, and how good both gym leaders are... Maybe if your partner was someone you knew in real life and you could hang out at each others place and run the gym, but when it's over the internets you have to worry about the other person not showing up etc etc...
Maybe if your looking for more battle options you could have 2 player Doubles, Multis, or even Rotation battles for those who don't play Singles well or want a bit of a challenge with something outside their comfort zone? :S