If PvE refers to leveling up and dailies, then a tank slows you down.
Unless you're planning on running instances and/or heroics, you don't need a tank really.
I'd personally like to always include a tank type toon, or at least a dps that can dual spec to tank, but that's a preference not a requirement for PvE teams.

4x Warlocks + 1x Druid is a very mobile team.
And they'd be very strong in battlegrounds too.
Not sure on arena, but Asonimie is playing 4x warlocks as a secondary team.