Beneficial to what! Haha, I don't think anyones ever asked that that I've seen, lol.
They are beneficial to your tanks clean up crew. They are filter feeders and any critter that utilizes food and converts it into harmless energy or breaks it down further than what it was before that critter got it, is considered part of the clean up crew.
Their numbers will not dwindle. They do however tend to pop up in the shade, maybe stronger lighting would keep them a bay, but that's a heck of an expensive change to make for that reason.