The previous posts addressed it well. A few additional things that I consider relevant is that the parks are a refuge for all members of society. Some visit the park and expect a pristine environment without man made structures. Or expect to be able to stroll without the need to jump off the trail as a biker passes. The speeds that are encouraged by jumps and drops are not always compatible with a mult-use trail. If it is a network that has an abundance of trails for which some can be dedicated to bikes, and one way in direction, then this is not an issue. You also have to consider the objectives of the land managers. In the WMA's, which are primarily for wildlife habitat, they are very conservative and don't want to see man made structures other than possibly a bridge which reduces environmental impact. State and county parks are much more open to user specific facilities but someone has to take the lead and encourage them. Speaking for myself, and the role I fill as a JORBA trail maintainer, my focus is making sustainable natural surface trails. I like an outdoor trail experience that uses natural features and this fits well with the constraints of the WMA land managers.
