A few reasons I can think of...

1) Running while casting.
2) Pets can tank, CC, and DPS.
3) Proven DPS against multiple targets.
4) Many CC options.
5) Easy classes to coordinate.
6) Can be 100% undead--WotF.

For the multi-box pvp'er, it just makes sense. I think Shammy's would work good too though. I seriously considered it.

I'm going to try 3 locks and 2 shadow priests.

pSon