According to a recent post by Ghostcrawler there are three primary reasons that they killed off the 2v2 arena bracket this season.
First, the most successful compositions were dominated by classes that were "well-rounded... with lots of offense and defense..." Think Paladins and Druids. Ever try to kill a good Resto druid one-on-one? It can get a little bit insane. Same thing happens in the twos bracket.
Secondly, there exists a problem with healer balance. Consider the following scenario:
- A healer can't keep himself and his partner up in 2s, therefore the healer would never be successful in 3s
- A healer can keep himself and his two partners up in 3s, therefore would be over powered in 2s
To balance that scenario, the healing out and damage in would need to be equalized between not only healers and DPS classes of different varieties, but across different multitudes of each class, and then even against different multitudes of each class. Such a scenario creates a wildly complex three dimensional matrix of possible combinations, which hurts your soul if you try to think about it too long. This is a primary reason why twos were removed.
I should note that there appears to be a misplaced word in what Ghostcrawler said in his post, but the above is what he's getting at.
Finally, the third reason that Ghostcrawler says twos were removed was because of the fun factor. "...larger matches are more fun overall and we think we can offer a better game experience by focusing on them," he says. Since 2v2 matches have moved into the unsupported realm (like Windows Vista, /rimshot), players who enjoy them are able to partake in them -- but Blizzard isn't going to worry about their balance anymore.
And on that note, twos still do offer a great place to practice skills and work out strategies against other classes, perhaps in an unbalanced scenario as well. That makes it all the better so that when you do move into a balanced scenario, your skills and abilities are all the more honed.