I had a small Warn winch, mounted about 18" above the bed of my enclosed trailer.
With a 1500 lb. capability, it was enough to pull my 2250 lb. race car with no problem up a 15 degree incline. It REALLY helped with pulling a fully loaded large tool box into the trailer!
With an open trailer, it's not needed as much, but still handy. Make sure you weld the mounting bolts/nuts, or make it removable, or you may find the winch missing some day (don't ask).