I did a bit of research a while back, and it seemed to agree. Mosquitoes don't keep any other species in check (since their victims rarely die), and they aren't a primary food source for anything; bats will snap them up all night long, but they're still a minority of the stomach contents because they're so small. They are pollinators--I hadn't known that--but, again, they're not primary pollinators for anything, not like bees.
I don't recall reading anything specific about the larvae's importance in aquatic biomes, though, so the anti-mosquitocide guy may have a point.
Maybe they keep bats too busy to eat a more nutritious prey animal, and keep the population from expanding though malnourishment? Similar to the diet of a panda bear, though they unfortunately have no higher-caloric alternative.
I don't recall reading anything specific about the larvae's importance in aquatic biomes, though, so the anti-mosquitocide guy may have a point.