The chassis is weak around the area where the rear upper shock is attached. It is a common failure and does require welding to repair or replace the reinforcement plate from BMW. BMW has an upgraded part that is thicker. There has been a lot of discussion on this so, check the Roadfly archives. Some people do remove the old reinforcement plate before attaching the new one so technically I guess it could be said the tower has been replaced.
BMW Part numbers for the rear shock tower repair parts are as follows:
41 14 8 152 997 (Left)
41 14 8 152 998 (Right)
Stiffer suspensions systems will definitely increase the likelihood of this happening.
HTH
ED
MWrench