Well, since I haven't tried 5v5 arena myself, the only input I can give is that you have more chance of it working in 5v5 than in 2v2. Having played both a rogue and disc priest I usually laugh at any shadowpriest I meet due to the fact that I can obliterate them regardless of which character I'm on. I reckon that in 5v5 teams there's usually a spot open voor a pure DPS and little to no CC (read: shadowpriest or shockadin) class.