I'm a proponent for doing what you can. A dip might or might not help, but if you buy your corals from suppliers that keep corals and fish in separate systems or maybe even a LFS that keeps frags with a fish or two, and you dip, you might not eliminate all probability of infection but at least you can reduce the chances significantly.
When I pondered getting back in the hobby, I realized I couldn't do a perfect quarantine for everything and also realized that no quarantine system or method is really 100% full-proof. I figured out what I could do and am relying on reducing my chances of parasitic infections instead of trying to achieve quarantine utopia. The quarantine methods these very knowledgeable folks have outlined are truly the best methods. However, I don't like to see people throw up their hands and do nothing just because they can't or don't want to follow all quarantining best practices. There are things you can do to improve your odds, even if you don't do every single thing and do it all, "By the book."