I've actually had some more play experience that I think kind of points to the fact that giving everyone a minimal amount of mezz protection (+4 mag protection against stun/holds) isn't that broken. I played over on a couple of the other servers (they had either basic mezz protection or AV triangles) and with Defense amplifiers and nothing really became broken, I just wasn't randomly held to death and couldn't do anything. And recently on some teams while I was playing with my Dark/Dark Sentinel, I actually got mezzed through their level of protection while teaming.
I truly feel that some basic ability to mezz protect the squishier ATs would only enhance their fun level and maybe make them a bit more survivable without actually making them 'super soloing gods that break the game'.
People say that it would break the game, but I'm very sure it would just make the end level game more fun for more people and ATs. Giving people more choices to play instead of pushing them to play ATs with mezz protection (I no longer find blasters, defenders or corruptors any fun because of the fact that I'm a second class citizen because I of no mezz protection).