I do it. I have one who doesn't drive and doesn't do much biting rig work, so I started ponying her behind the cart. I have a driving Mini that no longer shows, so I use her as my "pony horse". It gives her a job and gets the other horse worked. This is this mares favorite way to be worked. She loves it. I do NOT tie her to the cart. And I always do it in the ring, never an open field...just incase. I sit on the lead, so if anything happens, it's quick and easy to release the lead. I also make her stay behind me and the cart, not off to the side. I don't want her getting her legs hung up in the tires, so it's safer for her to be behind me. She knows the routine and stays right behind me and doesn't lag behind or pull on the lead. For the most part, I never have to touch the lead. She keeps up when we are going and knows the voice commands, so she is as quick to respond to whatever gait I ask for as the driving horse is, and she stands patiently next to the driving horse while I hook to the cart.
I guess it depends on the horse on if it'll work. I have another mare I tried to pony. She wanted no part of it. And I have a few who find it very boring. But, the one mare I do it with all the time loves it.
~Jen~