People just don't like how they look crawling all over the glass. They can carry parasites too, especially leeches, but once they're in the tank their reproduction won't add anything. If you introduce them intentionally they should be quarantined as you would a fish if you're worried about diseases.
I love snails, but my fish keep eating my babies
I just made a 5 gallon tank with crushed coral sand to breed them and hopefully add them back into the big tank when they're big enough to avoid predation.