I want to add that I'm not in any way trying to diss the roll film holders - the truth is I've been tempted so many times to buy one, but held out as I couldn't justify it. So I suppose I was genuinely curious as to how others used them.
I suppose I'm thinking that for me, a "local" trip for some landscape work to the nearby mountains will still burn $25 worth of petrol. Hence my comments about the costings. I really only work with b&w film that I develop myself so costs there are low anyway (s big bottle of rodinal, use at 1/50 lasts quite a while). I hope that goes a bit of way towards explaining the reasons for my questions.
The answers given so far have really shone a light on the use of roll film backs.