Condition mesmer in zergs?
You are correct, cleanse makes it difficult.
My experience so far has been that condi mes is most useful at the edge of zergs, as zerg fighting is not always just a smashing of team against team, and there is plenty of skirmishing first and around the edges of the battlefield.
As far as things you can do to be useful inside the zerg:
Time Warp
Feedback
Focus reflects
Shatter conditions = aoe condition spread
Ineptitude + Blurred Frenzy + Being in the middle of the enemy zerg = CONFUSION AND BLIND FOR EVERYONE!
Boons for fun and profit.
Also, while cleanses are common in the zerg, every condi you apply adds to the mass that cleanses have to chunk through. If your 8 stacks of confusion, 8 stacks of torment, and 6 stacks of bleed soak up enough cleanse that the necro’s chill doesn’t get cleansed for 3 more seconds off 5 targets, that’s 3 seconds of nerfed damage for those targets.
And if it’s your 8 stacks of confusion that survived the cleanse, you get to laugh while the enemy Ranger rapid shots himself to death.
The key, I think, is frequency of application more than raw duration. Get as many stacks as you can as often as you can.
Lastly, while those mender-killer Mistrust builds are kinda useless in most content, I could see it picking up some serious value in a zerg. It’s not hard to find a target that’s channeling in a zerg fight, so it should be pretty easy to start spreading some aoe confusion that way.
All that said, you’ll probably be fighting an uphill battle in terms of damage because of all the cleanses.