Indeed, the more 2on2s you play, the more accurate that 2v2 quality would reflect "your" 2v2 quality. But for a really accurate number, you'd need many 2v2s with many different people. However, as Veg mentioned, the "quality value" players have now probably isn't extremely accurate in all cases, either, yet it "approximates" it's "true" value once you play more and more games.

and it's not like quality, or more general, the point system isn't "abused" or that people don't "optimize" their games