I don't mind the "heaviness" of Linux on the RPi but I cannot rely on its user-space interrupts for GPIO use from userland. Instead, I have to write kernel software or give up on guarantees. So far it hasn't been much of a problem, but if I wanted to do more tightly timed things, I'd need to use a real-time OS.

Meanwhile, it's nice having the support of lots of Linux software so it's hard to justify switching to a non-real-time OS with much less support.

I've noticed, browsing the RPi forums, that people are disappointed that they can't get real-time behavior from Raspbian.