With regard to the question of keeping the fish in a hospital tank after meds are administered, I think that would depend on how well controlled the ammonia and nitrite levels are in the hospital tank. If the meds cause a mini-cycle in the hospital tank, it may be better to put the fish back in the main tank soon after completing the meds, because you're balancing the stress of transfer and low immunity against the stress of the conditions in the hospital tank. Behavioral considerations also can apply here for sociable and schooling fish.
I am not convinced that a fish would become dependent on meds, but they can kill off normal bacterial flora (gut, skin, mouth, etc). Antibiotics may increase susceptibility to other bacterial conditions immediately after the end of the course of treatment. Anti-parasitics and anti-fungals are more likely to weaken the fish through toxicity mechanisms and leave it more susceptible to subsequent infections.
All in all, I think it sounds like a pretty good idea to leave the treated fish in the hospital tank if the water conditions are good, at least for a few days to monitor health and allow recuperation. Good point, it is not something I had really thought about.