This might encourage DPS roles to not be such mindless drones then. It's true, most DPS just point and attack, but anyone who has been in a group with me will know that I give help whenever it is asked for any role. I pay attention to what everyone's job is during any dungeon, so I can easily give information to a first-time or someone who isn't quite clear about what to do. Also, (as nicely as possible!!!) if we wipe a few times on a certain boss I will offer tips to help whatever the problem may be, and will willingly ask if I'm the reason it's happening.
Also, I doubt it will be mandatory to vote for someone. I think you are overestimating the generosity of the general public. If a tank is just doing his job (marking targets, not attacking slept, etc.) they won't get an MVP vote from me for that. That's kind of what he/she is supposed to be doing anyway. Now, a tank that is getting blindsided by other idiot DPS and chasing hate down all while preventing deaths in a dungeon would certainly be worthy of my vote.
I think, if anything it might be too tough to get an MVP at all, versus votes being given out to a single role with no distribution. I like the system, though. It will make more people strive to be better players than it will discourage them.