At the end of the day, my biggest concern with warlocks as a team is that they are a cloth wearing class. You're going to take two kinds of damage in this game, magic and physical. With a few minor exceptions, everyone has pretty much the same degree of defense to magical damage, but physical resistance is alltogether different.

Multi-shammies have always been nice because mail+shield gives darn good protection against physical attacks, same now with the popular DK+Rets combo. Being a clothie makes you vulnerable to physical damage.

I watched a single bladestorming warrior decimate the multi-warlock team that run in my battlegroup. It wasn't pretty. As I mentioned earlier, it was a piece of cake for me to DG one warlock and kill it in 2 GCDs, then snare another and do the same.

Personally, I think an all warlock team sounds like a fun, fun time. I've been tempted to roll one myself in the past. I just fear that they're not robust enough to deal with the massive amounts of physical damage that other classes can easily put out. These same classes all have tools to close on you (charge, DG, stealth, etc etc) before you can kill them. You just can't keep foes at range... A lesson I learned painfully with hunters.