There's any number of perfectly legitimate reasons to ride on the road. Road-adjacent MUPs are for all intents and purposes sidewalks. And several studies have found that sidewalks are more dangerous than roads. If you pay close attention to cyclist fatalities, you'll likely notice a pattern of behaviour, including sidewalk riding, that seem unusually common amongst those killed. I'll ride road-adjacent paths only if intersections are few and far between, such as a path between the road and the river, or along a train route.

I've also seen some completely ridiculous paths that add 30-40% to the distance by meandering. I'm not going to ride that s***.

Other routes may be longer. Personally, the usually smooth pavement, better scenery and most importantly, quiet, of a path is worth some extra distance to me, but not every day, and not if it adds substantial hills, or more than 10 minutes or so to the route.