Cristian, I do agree with you about getting the right fit, and if you're gonna do something like this then you should do it right. Not to the Nth degree or anything daft but I still think that getting it right applies equally to mtb's and not more in favour of road cycling.
Let's talk about the more serious cyclists not the leisure cyclists, the ones who go out to enjoy and
increase fitness. I guess your saying that because mtbers on the whole do shorter mileages than their roadie counterparts that they are spending less time in the saddle and so it's not really going to matter? Not necessarily true, the terrain that mtbers travel over is slippery, bumpy, gnarly and loose etc, it can take 2 hours to do a 25-30 mile ride. Roadies on their smooth road surfaces with their skinny tyres will do well over 40 miles in that same time-frame.
When I go out on my mtb it's for at least 20 miles and as much as 50, I average more when I'm on the road bike (Yes, I know others do double that!) so I should really get the fit right too.
Let's take your point of moving on the bike. You may be shifting position here and there but in the main you are still sat on your ass and that's when any bad fitting will start to take effect on your body.
Personally, when I'm on my geared mtb for instance, I never (well, hardly ever
) get out of the saddle. On the roadbike it's virtually the same apart from hills as you so rightfully say. When I'm on my singlespeed well that's another thing altogether, you have to stomp when climbing the bigger hills or get off and walk.
True, you are more stretched out on a roadbike so that has got to be right, likewise most mtbers don't want to be stretched out over the TT so that has to be right too.
I'm not having a go, you've put your point across and this is mine.