Here is my answer:
Many of the riders you see on Seminole Hwy are going too fast for the path. Paths are not just for bicyclists, but also walkers, joggers, kids on bikes, dog walkers, and slower recreational cyclists. So if you want to go fast, either because you are in a hurry - like so many motorists - or because you are trying to get your heart rate up, the road is the proper place to bike. In the same way motorists cannot drive 55 on many narrow roads with poor sight lines or other, slower users, bicyclists should not be going 20-25 mph on the path, even when there is no one on it.
The bicyclists on the road also may not be coming or going to a location that makes the path a logical choice. Leaving the Arboretum, you use Seminole Hwy to cross the Beltline and get to many places in Fitchburg. The path crosses in a different location. Much as a bicyclist might ask, "Why is the motorist not on Verona Rd?"
The road is only unsafe if the motorists do not wait until it is safe to pass the bicyclist. I have been riding on Seminole Hwy for 20 years, and the only problems I have are when drivers are in too much of a hurry, and cannot slow down until there is a gap to pass.
All these perceived conflicts could have been avoided if Fitchburg had added bike lanes all the way down to Whalen Rd a number of years ago. But they said, "There isn't that much traffic. Bicyclists and motorists can share the lane easily." So now we have bicyclists doing exactly what the planners suggested, and the motorists being upset by that action.