Well, I actually ride a road bike with disk brakes (technically a touring bike, I suppose). It doesn't exactly inspire a firestorm, though.

Part of it strikes me as similar to the disk-vs-drum brakes on motor vehicles. While you can come up with reasons for using one over the other, both work okay, and neither seems to be so dramatically better as to drive the other out of existence.

I did the 12-hour race at the Texas Time Trials a while back. Three laps of that (77 miles) were in the rain. After the race, I overheard one fellow complaining about not wanting to be going downhill at 30 mph when he didn't have any brakes. So that was one of the few times I actually saw an advantage to them, as I just didn't notice any difference on braking.

One thing that is not obvious is that there are different brands and types of disk brakes, and different size rotors. So just because the general design has the capacity to absorb more heat in a descent, doesn't automatically make every disk brake better for that.

The brakes on my bike work fine, but they do require periodic adjusting. You notice the need for adjusting when you go to stop and seem to lack braking power. So properly-adjusted disk brakes may be more consistent, but you can be riding along and it suddenly dawns on you that your brear brake isn't doing much of anything, too. Not something you want to discover when you start down a mountain pass.

One things I notice with disk brakes is that the working parts are kind of all concealed, so it's hard to tell what condition things are in, or what needs adjusting. At least with rim brakes, the things are right out in the open where you can see what's going on.

I don't know how big of an issue the spare-wheel idea is. The rotor itself is really the only "extra" item required on an extra wheel. It does limit your choices of hubs, though. The brake caliper can require re-centering. This is not an issue when you remove a wheel and put it back on, but it may be an issue if you're swapping out wheels- I'd let you know if I had a spare wheel to swap out.

I wasn't really aware that anybody used tubulars with disk brakes. Sounds like putting slicks on your jeep to me.

All in all, having used disk brakes for a year now, I really don't see any reason to avoid them, but don't see a whole lot of reason to choose them, either.