The post I was planning to do was getting much to lengthy, so I decided instead to post the link to an interesting video. The vid should back up my point better than I could do with explanations. It's a vid of a huttball match.
(Please note that I don't know any person involved with this video, but I think that the person, who made it, is a great player. I learned one or the other trick from his vids.)
The point I am trying to make is that, when team A scores and team B controls mid, team B has a very high chance of making a counterscore. The separatists have a player who can stay at mid (meaning that he doesn't move beyond the acid pools) and still assist defending/attacking, namely Durge. So the separatists barely lose any defensive power (except near the goalline) while controlling mid, while the rebels would lose an important player in the offense/defense unless they send R2 to mid.
Not related to the vid:
The arguments for R2's combat strength brought up do not convince me. He doesn't have that many tools that can be used in open combat (It is new to me that he has a smoke screen. In episode 5 he created one using the gas ventils on Bespin. It's not something he can do anywhere.). And I still claim that the separatists can spare two commando droids to hunt him down. Any of the 'players with names' on the separatists side can quickly defeat the alliance forces, so the two missing commando droids will make up for the lack of additional firepower by keeping R2 away from mid.