Death Knight brings +13% magic damage for the entire party, an anti magic zone, +86 str/agi for the party, ability for the entire party to walk on water at any time, an immobilize effect, 2 silences, 2 taunts, 3 AoE abilities, etc etc etc

This is all PvE type stuff though.

I already have the DK at 65 anyway, the 5 shamans are at 70. But I really don't want to switch back and forth between 5 shamans and 4 shaman 1 DK, I want to make a decision, make it final, and keep that 5 man team for good. Just a tough decision.

One thing that might affect it... if I were to say stop multiboxing right now and play solo for good, I would never use my elemental shaman. Just a boring class solo IMO. If I spent months or a year+ making my 5 shamans awesome (including my main shaman) and then cut back on wow and stopped multiboxing, would I keep playing a single shaman? No. Would I keep playing a DK if the DK was my main? Probably.

Thats a pretty big thing there I guess...