Are you actually trying to refute the definition of a word when the actual dictionary definition was posted right into this thread? You're not that dense are you?

Synergy:

1.combined action or functioning; synergism.

So you're saying that 2 classes that have a LOT of spells with the exact same cast times and effects that go well together (different dots, etc) don't fit into the "combined action or functioning" definition?

Riiiiight.

Honestly, thats closer to the REAL definition of synergy than a damage bonus from one class to the other is. In fact, druids and shamans fit the "synergy" definition far less than a shadow priest and warlocks, regardless of the damage bonus.

This entire argument is aside from the fact that with a priest, whether its shadow or not, you can heal your other toons, with 5 warlocks you're completely gimped in that area.