The original concept is that of the linear park -- a destination in the city or suburbs where locals of all ages can go get fresh air and exercise in a natural setting. Mellow recreation was the idea. The bicycle has always been part of this mix. But MUPs aren't always simply about recreation. The use of these paths as transportation corridors, rather than parks, is being pushed increasingly at a local level, and even promoted by the feds, including in a recent interview with U.S. Department of Transportation Secretary Ray LaHood on this site.
But there's a disconnect between the speedy reality of bicycle transportation and the slow, recreational uses these trails are designed for. Multi-use paths are only required to be eight feet wide. That's fine for a stroll in the park, but when you factor in two lanes for bikes as well as joggers, skaters, and roving families, it's alarmingly narrow.