It's caught my attention again, though I think I've made a thread like this before.
Pretty self-explanatory, it should be compulsory and in the rules of RfA to leave a reasoning when voting for a User's rights nomination, since pretty much everyone only leaves a signature and that can lead to bias/not really caring/just for the sake of it/or not being sure enough.
As this a website, hefted with rules that people should take care of, making it mandatory to provide reasoning when bringing such a change is only a benefit and not much of hassle. You need to know why the user supports or, especially opposes to get a more reliable consensus.
The least a user can do is just point out the things in the nomination para they like.