They keep the critter count low(flatworms, bugs, isopods, etc.....)
Agree. If it is a frag tank you want only livestock that can benifit the growing of your frags. Anything that does not provide a service to the frags will only add to the organic load and even possibly disturb them.
Just a FYI - but think utilitarian. Snails and hermits are usually toppling over my frags so their out in a frag tank for me; but a diamond watchman goby keeps the sand bed surface clean and sifting sand puts particulate matter uo into water flow for dispersal.