One significant drawback is that many of the bikes used for touring don't have the lugs required to fit disc brakes either on the fork or the rear left dropout.

I have just acquired my first-ever pair of disc brakes for an MTB project, and pair of Hayes MX1 with cable actuation. I have the fork to fit them, but now I am perplexed by the fact that the frame (yes, another dump retrieval, but an OK one) doesn't have the rear lugs, and I am looking at acquiring a new frame that does. But then I could just go with V-brakes on the rear. Decisions, decisions...

Having just set up the front hub on the fork, I found the adjustment to get pad clearance to be extremely simple. As to on-the-road adjustment to avoid rubbing... well, I don't think it would be any more difficult. Adjustment of hydaulic calipers might be different.

Carrying two or three pairs of replacement pads also would be nothing compared with pads for cartridges for any rim brake.

The benefits were outlined well by Neil. I think that as far as the problems go, you only have to look at the experience of the MTBer. The drop-out issues were major ones, I think, when poorly designed forks were placed on the market. But they have been redesigned, I believe, so the leverages on the axle won't pull the wheel off. Looking at my Magura Odur, the drop-out itself has a different angle, there are lips around the dropout, and the caliper mounting tabs obviously are placed behind the leg. Of course, the through-bolt system has now been adopted to make absolutely sure wheels won't come off.

The issues of torsional stress on the spokes I don't see as valid when compared with the stresses of normal riding uphill in very low gears with a heavy load. As to stress on the hub and bearings, that might be a different issue.

The only major drawback I can see is damage or distortion to the rotor caused by a fall against a object.