I feel that a strut bar is a benefit for even stock cars. Steel is a remarkable material in that it can flex below a certain limit an unlimited number of times with no fatigue.
What that limit is will depend greatly on the specific material and it's temper or treatment.
Flex more than the limit, and fatigue will happen, related to the amount of the flexing and the number of times the limit is exceeded.
Unless you drive in a most un-BMW like manner, a strut will reduce the body flex when cornering. We don't take our cars apart, dip them, replace large sections of the body, redo the leather and get supreme paint jobs just to have the car last a few years. We expect that they will deliver the driving experience that we enjoy for a good long time.
Adding a strut bar will help stiffen the body which will reward you daily, and down the road as well with a body that is still tight and in better shape than otherwise.
Difficult to quantify exactly how much difference it might make, but I believe it's a good value for the potential return.
Ian