Thanks for all the replies! I don't remember reading a particular article, just something along the lines of "if you have lots of fish waste, you should get shrimp and snails." I don't have algae yet and there is never any leftover fish food (unless I try to feed veggies, they are ignored), so I thought that sounded kind of fishy. It sounded too much like the old spiel about how you didn't have to feed catfish.
Of course, now that I have started reading about them, I kind of want some anyway! My air pump can run a second sponge filter and I have a spare 5 gallon tank ... I bet I could raise duckweed along with them ...