Its a very personal thing how you want the overall balance of the car to be.
By introducing additional roll resistance to the rear, you will make the rear grip less relative to the front, as such the rear will be more inclined to break away.
If you drive style suits having the rear swinging around and your car currently doesnt do it as much as you would like then buy one, if either you dont like a loose rear end or your car already has one then dont bother.
Personally with your current setup I would fit one, but I come from a largely RWD background so quite like a loose backend, you however might hate it.
So really like
Chi says, its a case of try it and see if it suits you or not really.