I'm still using the multi volume support in tar for example. Which was something that stems from the time when tar was used for tape archives (hence the name tar) on actual tapes. Without that I'd be really screwed because I use a box full of harddrives as backup "tapes" (which works surprisingly well I must say, I needed a small restore only a week or two ago and it really saved my bacon). But I bet 99.9% of tar users have no idea it can even do that.
Rsync is another one of those swiss army knives that people use for a lot more stuff than you might expect. Especially the remote capabilities are amazing.
The problem is that when you clone something but don't provide full compatibility, you're putting your users through much headscratching and frustration. It would be better to not name it after the original then so it's clear it's something different.