In Fort Joy high judge Orivand transforms a Sourcerer into a Silent Monk instantly just by purging her with a wand. So I don't think it actually requires mind maggots. But since you can craft a mind-control grenade from them, maybe Magisters just use them to turn Silent Monks into Silent Watchers? Most of the monks in Fort Joy are passive while the Watchers are actively fighting for their masters, and maybe mind maggots are needed for that.