I'm not sure whether this has ever been discussed, if not, here it is :-)
To me, it's one issue that should be resolved in smart trackback, or even when following planned routes, In the current way this works, when you have passed a specific trackpoint or waypoint, you have to press a button to make the arrow point to the next trackpoint (so telling the device: "ignore the trackpoint that I missed, just go to the next one)
In mountainbiking, there can be many reasons why you cannot follow the exact route and "hit" your exact trackpoint. Even bag GPS-signal can account for that. But when cycling, you simple cannot start pushing buttons to force to point to the next trackpoint. It needs a better solution for that to work automatically.
Have you ever thought about implementing a feature that shows an compass-like dial (just as the one GS already has), but underneath the arrow, the screen also shows a vertical line that represents the direct path between the two waypoints you're currently connecting. That line should move left or right of the center, indicating when (and how far) your are deviating from the planned track (not inventing anything new here, Garmin has such screen). Therefor, even when you are (or have to) deviate from the track, you would still have an indication on where the track is in combination with the compass-dial information on the next trackpoint.
The combination of all of these above features could proof to be a 100% workable navigation feature without having to switch to any map-mode.
I'd love to field-test the device also!