The good, the bad, the ugly...
Zerging, or completing the quest in a minimal amount of time works fine when everyone in the group either can do it well or can do it well enough to at least keep up and stay out of trouble. When some of the group try to zerg and can't manage it, that leads to reduced xp and/or resource consumption. The ugly is when a zerger joins, takes off, is no support to the rest of the party, and lets the party deal with dungeon alert.
As long as the LFM states clearly what is expected and the joiners know what they are joining, there is plenty of room for all types.