I'm no offroad guru but I have a reasonable idea of what will, and what wont work on the rough. In my opinion, a 120/70 17" tyre is only good for looking good!
What you really want on a heavy trail bike, is to have the ability for the front to be narrow (to dig into the terrain to allow the thread to bite), to be of a high profile and run a low'ish pressure (this will give the tyre a good chance to mold its self to the terrine without causing pinch flats). Hence the reason why most true offroad rubber is 110/80 or even 110/90
120/70 will only float over the rough, and the profile is such that (once the going gets rough or the speed starts to build) pinch flats will become inevitable.
what I don't understand are 2 basic facts: (1) there are loads of options for a 17" rear but no decent profiles for a 17" front and (2) so many manufacturers are now producing touring and dual bikes with 17" font and rear, that I just can't understand why the tyre manufacturers haven't caught on ... WHY?