Real quick and simple answer. It's bad, nothing good about them, zero benefits, you want them gone.
Some fish will eat them, but this is a poor solution. For one, the fish in question invariably get large or will eat other inverts. For another, they are messy eaters. They spread bits of aiptasia all over. You may not see any complete aiptasia while the fish is present, but once the fish is gone, the aiptasia will be back ten times worse than before.
If you only have one or two, the best bet is to just remove that one anemone either by removing the rock and burning the anemone off, or by using a syringe to inject with aiptasia x, Jo juice, lemon juice, or boiling water. If you have a few small ones, peppermint shrimp MAY eat them.
For severe infestations, berghia nudibranchs are the best solution.