I still think this about covers it. A well trained dog won't:
-stop dead in the trail
-run zig-zag across the trail
-poop on the trail
-chase other people or dogs
-get in anyones way any more than another rider would, which is inevitable with more than one person on a trail

I guess when it comes down to it, location is the biggest factor. How many people will be at your local trail on any given day? Because for me, there will be half a dozen people on a very busy day. On a normal day, one or two people at most. If your trails are popular, it's probably best to leave the dog at home. If your trails are like mine, I see no problems at all.