About 50% of my commuting is on MUPs.

I can do a route that's about 80% MUP--it's actually shorter than any all-road route (screwy road alignment, parks, etc). It's plowed and salted and, so I take it all winter.

I do mostly road riding in the summer, but still take a bike/pedestrian bridge and short chunk of MUP rather than riding through a busy cloverleaf interchange during morning rush hour.