The biggest issue I always had with the dummy rounds is, its very hard to tell them apart from live rounds, especially when they are in the gun. The chances of a mix up are just way to high and easy.
Another issue with the dummies is, they dont hold up very well or long, and soon are trouble themselves. Constant chambering, ejecting, and falling to the ground and bouncing off things, just beat them up. Most notably, the bullet set back (and no matter how hard you crimp them) and the rim of the case being chewed up by the extractor. With the better snap caps, the rims seem to hold up a lot better to abuse, and just due to how they are made, you dont have the setback issue.