I've used a 2500lb warn ATV winch, which worked with a snatch block OK, but I wanted more. I'm putting a cheapie 8k that I have from a Jeep I bought on there, which should be fine.
The snatch block + ATV winch can be short on cable in a lot of situations, something like 20-22ft from the winch to the vehicle you are pulling, trying to leave enough wraps on the drum. With a full size winch, you can be 30-40-50 ft away with the trailer, and have someone steer the vehicle quite a bit. I've been in those situations where you can't or don't want to get the trailer right up to where the thing is that you are trying to winch.