The videos I have enjoyed the most were filmed with multiple cameras. Watching someone ride a trail for 30 minutes, I'm not going to do it. Have different camera angles, and edit it down to the better shots that do a good job of conveying the overall feel of the trail,
Now, that said, I can totally see the advantages of filming the ride so you can see the areas you need to work on. Better lines, more/less speed for various conditions, etc. And by allowing others to view and comment you can get ideas and suggestions you wouldn't think of on your own.
If you are filming for training purposes, then whatever mounting point gives a good view of what you, the bike, and the trail are doing should work fine.
The purpose is different for each type of filming, so you have to take that into consideration.