I agree that it's possible - potentially trivial - for a bot to figure out if a field is hidden from view, but "really ineffective" seems a bit extreme for something I've seen work well multiple times.

It's not a definitive solution, but it's an easy and practically free first line of defense for a young project, and depending on the project, can stand for years.

Overall, it depends on the sophistication of the bots your project attracts.