Indeed, having the global rating optional defeat the design of a global rating
And it's true that if you play against a much higher rated team, you will lose barely nothing.
I think it's kind of broken because of autoteams, but you can try typing "get matchup" or "get quality" in the lobby chat, and check faf chat.
A quality index below 50% (or 0.5, depending of the scale) means a very unbalanced game.
A "any outcome is possible" range is beyond 75%.
You can try the trueskill calculator to see how any game can affect your rating. Of course, you don't have access to your skill and mean factor (your rating is skill - 3 * mean), but it can help understand the maths.
http://atom.research.microsoft.com/true ... lator.aspx
A new player got 1500 in skill and 500 in mean (resulting in a 0 rating).
Or maybe you can read that to have the math explained :
http://www.moserware.com/2010/03/comput ... skill.html