I think they're good for newer hobbyists, they help control parasites and increase water clarity. Once you've been in the hobby for a while and you've invested in some nice equipment, you water quality has increased to the point where parasites are no longer an issue. When the fish are healthy in a stable environment they seem to be able to control their parasite level naturally and prevent an outbreak.
At least this is what I've found. I also run Zeo, so I couldn't use one even if I wanted to.
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