It is an extremely poor substitute for a properly adjusted derailleur. They can also cause issues as they can cause a freehub to not function properly I have seen that a few times in recent years. Essentially those that believe that the cheap plastic disc that ends of cracking and making noise and looks like crap generally is important probably aren't knowledgeable in their bikes enough to know it is isn't. Limit screws work very well like extremely well and if things do get damaged in the end that disc is unlikely to stop much and your derailleur is going to move and likely get damaged or at least your hanger is compromised.