Thanks all for the information. It's starting to make a little more sense to me. Wider rear spacing of 135mm provides less dish therefore stronger wheel. Lower gearing options with a MTB crank.

I have a road bike capable of light touring but I'd like to build one for heavy touring so I've looked at a lot of photos of builds and the usual suspects seem to be some sort of Deore RD, low range cassette, either a road or MTB crankset probably depending on whether the rider is really going to carry a heavy load or are going to commute with a lighter load and occasionally take a loaded trip, front D doesn't seem to matter as long as it can reach a triple, 135mm rear spacing seems to make sense I suppose. Cantilever brakes are traditional on touring bikes, but I read an article in Adventure Cycling about the advantages of linear pulls. strong rims with lots of spokes. Except for the shifters, sounds pretty much like your average hardtail to me except for the longer chainstay for stability and heel clearance.