Some more thoughts...

1. PvP rewards are pretty much the best gear a multiboxer can get without the backing of a guild. Even Ellay has said their's nothing he wants in terms of PvE gear from raiding. So multibox specs are most likely to be based around what's effective in PvP.

2. 4 boxing+1 is very different from 5 boxing. 4-boxing+1 opens up more viable classes, since you are primarily focussed on DPS'sing your oponents down while someone else is throwing you heals. However in 5-boxing you're trying to do both DPS & healing, which is quite a bit harder. One of the main weaknesses in a priest+lock, or priest+mage combo in PvP, is that if your priest get's focus fired down, it suddenly becomes a lot harder for you. With 5 shams, losing one isn't as bad - you can still deal damage and heal effectively.

3. PvE is still important - for levelling, earning cash, doing something OTHER than PvP, etc. Shamans are the only multibox team (so far) that have had sucess doing heroics, other than creating a dedicated PvE team.

Cheers,
Stealthy