Don't expect to win any arenas with 5 cloth. It just doesn't happen.

Mail + shield + 5 healers + totems + NS + wolf form + LO + 5 anhks + 5 rezzers >>>>>> 5 cloth in arenas.

In AV? Mass world PvP (like the new siege battleground?).... 4 mages shine. 5 and no healers? Bad idea. But 4+ shammys have so damn much going for them in arenas that the combo just WORKS.

Since arenas are the new leet gear spot, most people are choosing shammys. Not to mention you really can't do heroics with 5 cloth.

If mages didn't die in 2 hits and run OOM so damn fast, maybe they would be viable in arenas. Until then, forget it.

That's why you see so few people boxing mages. Warlocks? Much more popular but DoTs can be dispelled and people with DoTs and pointy things don't stop poking you. 4+ Shammy is the best combo for arenas in the current state WoW is in. That's why they are so popular. Jack of all trades master of .. nearly all.