I think they are silly, no offense to those that use them. I read where people use them to get others attention or whatnot, but that can be done without these modes. I have also read where people could use them to temporarily incapacitate an attacker. Really?
I wouldn't be so anti-disco if they didn't take up a slot that would be infinitely more usable. For example, if I could program whether or not disco takes up a slot, and if I don't, I can use that slot for another brightness mode, that would be much better. I don't understand the seatbelt analogy. Seatbelts are useful, disco modes are not. Period. If you want to signal, you can use momentary-on, or you can twist your wrist with the flashlight on to get the same annoying effect.
Another reason I hate disco is that I have a light that has the disco, and I sometimes turn it on accidentally, and it just irritates me. It both annoys and at the same time reminds me I have lost a usable slot in the flashlight because somebody likes the bee gees too much.