The a/c system not only cools the cabin, it also cools the batteries, so if the compressor destroys itself and gets metal bits in the lines, it is a major issue to remove all of them (often by replacing parts, but getting to them is part of the issue).
So, probably only BMW has the real stats on this, while it can happen, doesn't seem to be a major problem. IF it does, it's expensive to repair. I suppose they could have made the two systems separate, but that would also mean more weight. Everything is a tradeoff.
FWIW, not all a/c fails would throw metal particles into the system, and usually (not always) that would be preceded by some funky noises or loss of performance that, especially in this case, should not be ignored! Even in the wintertime, to dry the air, the a/c system can run, so you'd notice it then, as well as in the summer.