i like doing threshold workouts on the treadmill because then i know i won't go too fast. and progression runs are a lot easier in the treadmill too. plus i find that treadmill in general can improve my mental strength because of the monotony, especially if i rawdog (no tv/music/audiobook/whatever else) it. those things are enough motivation for me.
but for the majority of my mileage (easy runs), i much prefer the roads, unless i have a really good audiobook/album/show/movie i wanna consume, in which case treadmill might be fine too.