grumpus

Because it was too stiff, or because it changed the steering geometry?
That's great if you want to lock up the rear wheel while not slowing down much.

Aubergine 

I'm honestly not sure, but I suspect it was the geometry.

Can't see why I would be any more likely to lock up a disk than any other brake. As for slowing down, I can attest that the rear brake on my disk brake bike offers a decent amount of stopping power. Not as much as the front, but it definitely works, and better than rim brakes when it's wet.

grumpus

If you're braking hard on a good surface the front brake does all the work, to the point that maximum deceleration is achieved by barely lifting the back tyre off the road. Presumably you're fitting a disc brake to improve this ability, unless you're heavy touring or riding a tandem in hilly country in which case a drag brake may be useful. Sure the back is useful on less ideal surfaces, but it's really not going to stop you in a hurry, and any attempt to do so is likely to test the tyre's grip; a good rim brake should be able to exceed this already. Riding in wet weather? Use good brake blocks and wipe the rims before you brake harder.

* I'm ignoring off-roading, particularly DH MTB, as this discussion seems to be road-based.

Aubergine 

I question this. In all my years of riding I have never lifted a rear wheel. But then, as a skilled rider I know how to throw my butt off the back of the saddle to weight the rear wheel in hard braking.

It's the need to wipe the rims that disks avoid. That extra time makes the difference, even with good rim brakes and Kool Stop salmon pads.