Power Vs. Condition
You can only stack so much conditions and there are better condition classes.
Its really quite simple:
You join a group that now consist of 3 Mesmers and 2 Necros (yeah a little extreme lol).
Who do you expect to be condition specced and who do you expect to be direct damage specced?
If both of the Necros run conditions – which would not be surprising at all – 3 Mesmers running conditions will be completely ineffective. Basicly, you can only have a couple condition builds in a dungeon group or its pointless. People dont really expect Mesmers to run conditions because they are very good at direct spike damage by class design. In terms of conditions they are far, far more random than any other condition built class. Mesmer conditions is ALL RNG.
In open world PvE (ie boss events) the condition builds are even less effective.