One advantage I can see with a disc mower for roadside mowing is the fact that it extends far to one side of the tractor. That may well be the reason why a road dept. or county maint. crew would use one. My first disc mower was bought used. The original owner used it like a bush hog. It was 3 years old when I got it, and was for the most part, junk. Maybe I just have a bad impression of disc mowers for anything other than cutting hay (in a clean field)
You are correct in your appraisal of single vs. double spindle WIDE mowers. Once upon a time, I had a 10.5' Bush Hog 2-spindle 3-point mower. I now have a 7' Woods single spindle that I WISH someone else had. It grabs up rocks, scalps on uneven terrain, and is in general, just too awkward for most of my cutting. I recently bought a new Bush Hog 6' (286) cutter and let the Woods sit unless I'm mowing pastures. The 6'er works MUCH better.
Mowing rough, rocky, and hilly ground is a beast all it's own. Any advantage you gain usually nets a liability in another way. Fast is out of the equasion.
I'd still recommend looking at Rhino/Alamo/Shulte products or Bush Hog for their extreme duty mowers. Here in Kentucky, where rocks and hills are the norm, I see most highway dept. or commercial mowing contractors using one of those 2 brands for the most part.