The problem with having a trailer hitch assembly installed is that in a rear end impact, the bumper is designed to absorb the impact. If you have the hitch assembly, regardless of whether or not you have the ball on or if its just the receiver, the impact is going through the hitch into the structure of the vehicle where it is not designed to take impact.
Also, as was mentioned by the nissan tech that is on this board, the brakes on the versa can barely stop the car itself. Now you are loading down the back, which will make it considerably worse for braking as well, and adding that extra weight.
For a car as light as a versa, 1000lbs is alot to tow. If you get a gust of cross wind you are toast. If the trailer starts to wobble, you won't have the power to pull out of it.