The only real advantage I can see from a 6th acct (wish I had one for this reason) is buff chars.

I have druid, pally, mage and priest, but all are on one of the 5 accounts. In an ideal world, before a heroic, I'd invite the buffers 1 by one into a raid to buff the whole group.

I do this sometimes with the priest and mage, just taking 1 slot of the 4, but one char always therefore misses out on the buffs.

Hope that makes sense :O