OP just asked if they were viable.

We don't know if this is a PvE or a PvP server.
We don't know if they'll be questing up, instancing up, or running battlegrounds.

We really don't know what the question is in regards... viable for what?




If they're on a PvE server, then PvP is purely consensual.
5x Warlocks might be extremely viable for questing, dailies on the team, and battleground style PvP.
Possibly as DPS in LFR or a guild raid.
Pretty sure a healer (probably played by someone else) will be a requirement for success in arena 5's.

If they're on a PvP server, then they can get into combat at any point.
Sometimes they'll be outnumbered or against someone much higher level.




If the focus is on Arena, then the question of which viability still varies.
Is the goal to hit 1800 and to push for 2200+?
Is the goal to cap Conquest Points weekly?
With MMR, if you play on days when the masses are playing (Mon before server reset, Tue after server reset, Weekend afternoons), you'll eventually encounter teams at your skill level.
The system is not perfect, you do get teams well above and below you.

If the focus is on Battlegrounds, a much less optimal composition will work fine.

I've seen boxers post that they want their team displayed on Grid/Vuhdo/Healbot, but have no interest in displaying the rest of the raid.
Basically meaning, if they bring a healer it is purely for their team.
If your healer will help the raid/battleground out, as well as your characters then bringing a healer to a battleground is a bonus for others.