Unidirectional tires need to be re-mounted when swapped left-right. If you're running summer-only tires, they're probably unidirectional. Heck, even my snow tires are unidirectional.

Probably clarifying with the obvious, but eh, there it is.

 

^ Never hurts. Thanks for expanding on it.

 

tire rotation helps prolong treadlife by preventing uneven wear. It doesn't help with performance unless you have bald tires. I feel that after tires get rotated, you don't get optimal traction exactly because you are now trying to wear the tires on areas not being wore before. BMW does not recommend rotation for that reason on many of its models with aggressive rear negative camber. So, not rotating your tires is not a big deal and definitely not to the longivity of the car itself. Balance on the other hand is usually obvious in the fronts when you get steering wheel vibration but harder to tell in the rear.