from someone who considers himself to be an average cyclist (ride maybe 2-3000km per year tops) I really dont think this is an issue. Yes, as I mentioned before, I wipe the gritty gunk off my rims when I get in from being in the rain and clean my pads once in a while too, so this helps a bunch and takes only a minute or two when everything is wet (if you let it all dry, its harder to clean)
I realize not everyone likes doing this, but it really helps with braking power and rim life.
Yeah, as I mentioned I haven't been cycling regularly for a few years, but I was, I hate to admit this, but I was really lazy in terms of keeping my pads and rims clean.
That said, I've had the same front rime for 6 years, and even though it's close to needing a rebuild, it's lasted a really long time. I had to have the rear wheel rebuilt every 1-2 years: Since most of the weight is on the rear wheel, I'm not sure how far disc brakes can really go in extending rim life.