I have the perfect solution: I bought another mount for my additional 5200 mAh Battery and this way I can keep the display on all of the time

@menion:
I agree with Joachim that the Navigation feature is not really applicable for mountainbike purpose because it does not recognize turns in the right way.
However, I think that a small change to the Guiding feature could make it useful for mountainbikers who want to follow a track. There are are websites like the german gpsies.com. They let you create/modify gpx files and you can add "turning notifications" at certain points. So if the guiding feature could recognize those "turning notifications" and just wake the display up/beep on those points and not on all waypoints, this would probably satisfy most mountainbikers.
To implement such a feature, the track format will somehow have to support different type of waypoints or some additional data, to distinguish regular waypoints and turning points. I have no clue, if gpx supports that kind of information.
This way, you would have to modify the downloaded gpx tracks before following them. But I guess an automatic navigation based on the angle between waypoints will never get satisfactory results for the kind of tracks you follow by mountainbike.
@Joachim:
This leads me to the following scenario:
If I want to follow a track with the mountainbike I choose openandromap with MTB theme in Locus.
Than I add an new route within Locus following the loaded GPX track. This is 20min additional work, but I get a clean track on the paths and I get turn instructions only when I need them.
I am also interested in what you mean by that. Probably a similar thing that I described... modifying a gpx track by adding manual turn notifications.. just in locus and not on a third party website. But its just a fictional scenario, so you cannot do that by now, right?