Does the PREEMPT_RT patch (real-time kernel) have any benefit for regular desktop users?
I don't think so. The patch seems to provide real-time scheduling which is very important for some enviroments (planes, nuclear reactors etc.) but overkill for regular desktop. The current kernels however seems to be enough "real-time" and "preemptive" for regular desktop users.
It may be useful if you work with high quality audio recording and playing in which even small amount of time may dramatically reduce the quality.
 Technically both are 0/1 features but I guess it is clear what I mean ;)
I'm guessing you misunderstand the concept of 'real-timeness'. If not, sorry, but it happens a lot, and I thought I'd throw a little clarification in here.
The main point of a real-time kernel is to serve requests within a predictable deadline. That does not necessarily mean faster than a 'normal' kernel. So for desktop systems, a preemptive kernel is good, a real-time kernel much less so.