I think its fair to play a class you're still working on. I know I've been forced into a roll I dont know well regardless with some teams comps. I definitely agree its better "training" as the competition and coordination is better/focused.

I play Rhein quite a bit. He is all bout tactically keeping a charge on your shield (dropping it enough to keep a good charge).. A bit moreso than Dva as you dont get the luxury of the ranged attack. I'm always trying to find a position where I can duck in and out of cover. On Defense I like to cover the main choke point off to one side, and try to do a charge across the front of the choke when attackers try to push through. Instead of pushing straight at them where you will ultimately range your healer.. If anyone gets close its swing away while trying to juke around and not take too much damage yourself.

I climbed 100 or so points over the weekend. its not impossible.