At night in rain the mirrors are not sufficient. Turning to look won't take into account a car that moves quickly after I've looked into the next lane to me. I recently drove my wife's Lexus for a few weeks. BSM is just safer.
As someone who drives more than 100k miles a year, I feel I can offer some insight on this. Whether you agree or not, is another story.
I view BSM, cameras, and radar are effective TOOLS, that can HELP you accomplish your task, which is driving safely. I personally don't ever rely on things completely that can make me more complacent in my driving. Having said this, I have driven vehicles with BSM for the last 8 years. I can honestly tell you, these tools can be fooled. I saw someone posted that they had a lot of false alarms. I can attest this is true, even on newly designed and released systems. I can also tell you, that my company no longer buys vehicles equipped with BSM.
This isn't saying people can't have or use these tools. Saying BSM is safer however, is, IMHO, a bit of a stretch. At the end of the day, the best tool in your arsenal, is above your shoulders.
I personally would not spend the money on such a system. But again, just my .02