I try and pay attention to people who might not have high stats that contributed however failing that I tend to give a look at all the stats and see who seems to have stood out.
I have no set rule as to who gets it. IMO Damage, Kills, Heals, Protection, and Objective can all have equal weight. Depends on the match type however saying any one type of contribution is more important (seems overwhelmingly people say heals) is rather foolish. What about the tank that guards the healer so he can do that healing, or even the dps that focus fire to kill the target that is focusing the healer. Heck even aoe damage to prevent someone from interacting with an objective I could go on and on. All these can be just as important as the other.
The fact is im not that concerned that I didnt get a vote. There were others that also deserved and gone none while one person gets 3 simply because he was on a team with three friends. I would have been content to get none if the votes had gone to others that deserved it.
Now as a constructive suggestion rather than removal. One that has just occured to me. Instead of voting for your own team you should be voting for the opposing team. This way it would be far less likely to be able to be rigged. Sure it might happen occasionally but at least it wouldnt be consistant.
Regardless of what they do I will continue to vote for who I belive deserves it. Sometimes that might be a friend I qued up with but often it will be someone else who stood out.....and not always just in the stat block.