I have been giving this a lot of thought. I think mmr should be simple and easy to understand and, most importantly, make sense in light of how bgs will likely to continue to play out in this new format.
Rules:
1. You get 2 points for each kill that you secure.
2. You get 1 point for an assist because it shows you aren't standing on a rss or capture point holding l2. You are actually trying to win. This part is controversial but the reality of this is very clear when you extrapolate to its natural terminus. If everyone stood on resources and healed or blocked the flag wouldn't move. You have to try to kill...
3. You get 2 points for having the highest heals on your team and 1 for second here.
4. You get 5 points for winning a bg because if you did 1 and 2, and likely you WILL WIN THE BG. However, if you are a really good Xer but get the short end and get comped with less talented group mates your mmr will not suffer as much. Even if you don't get the win bonus.
The above is not to say that tanks do not have their place but bgs is not the place, imho. They make bgs less fun and do little to nothing to contribute to the success of a team in a bg.