[MOTORHEAD]

If you have a "significant other" they will feel that you don't care about their safety if they don't have a rollbar.
If you want to track it and have to start from scratch and have an instructor ride with you, they won't do it w/o a roll bar on their side .
If you don't have to deal with either of the above, then one is all you need.
And they are not just for looks, I have seen a few shiney side down !

Ted
ps: I am planning to add one to the pass side or go with one full width for both reasons stated above.