Well they do use rack and pinion on the BMW 5 series after 89 so I guess it could leak onto a boot over time and cause it to go bad allowing open up and cause foreign objects to get in.
Keep in mind one components failure could cause others to go as well. It's like replacing control arms but leaving bad shocks on. You just are going to kill the control arms.
I don't know the specifics of the newer BMW's but I would go to a good independent shop and get another opinion. Also post on Roadfly.com
If he is out of warranty why does he still go to the dealer?
my 2 cents