If it's like other paths that I've seen, I'd guess that what you call the "jogging lane" is clogged with slower-moving foot traffic, dog walkers, people pushing strollers, and the thunderstruck minions of the cellphone who screech to a halt wherever they happen to be and carry on their conversation, oblivious to all about them. If that's what it's like, I can understand why a jogger would find it unusable.

Locally the MUPs are often have physically separated bike/skating lanes and pedestrian lanes. They typically merge and cross. They'll be shared for awhile and then they separate again. Often times the walking/running path will be the more scenic of the two, - closer to the lake, river, etc. It also often doesn't "flow" as smoothly and will take a more erratic course around trees and structures.

Given that it's easy for a jogger to inadvertently end up on the wrong path. It tends to happen less on the busy ones because newbies end up following people who know where they're going.

I'd also bet some people run or jog in the MUP bike lanes to avoid the slower walking traffic in the pedestrian lanes. I'm fine with that as long as they yield to cyclists.

As far as bike lanes on streets go, I don't see many joggers in them. I do find myself jogging in them once in awhile out of habit because I'm used to being on my bike so I naturally avoid the sidewalk. I have to say I much prefer running in the bike lane to the sidewalk.