Yes, I like the concept. I also like the Concentration mechanic as a limiting factor. RMU gives very concrete rules for how Concentration works. It also makes for a fun tactical challenge, whether the summoner is a party member or a bad guy: the summoned creature is strong, but it costs the summoner half his activity for the turn, and if you can hit the summoner hard enough, the control breaks. That adds not just balance, but some dramatic tension!