I believe that BSD or any other OS for that matter, is important for two reasons:
First, to find performance issues in Linux. A couple of years ago, some guys benchmarked FreeBSD against Linux on MySQL OLTP and Linux scaled badly compared to FreeBSD which turned out to be lock contention in the Linux VM. The result of this comparison was improved Linux performance. Does anyone remember the Mindcraft benchmarks from 1999? These were a wakeup call for the Linux community and eventually resulted in much better scalability.
Second, there was a time (late 90s / early 00s) when the FreeBSD VM *was* superior the one found in Linux. There were even exchanges between Matt Dillon and Rik van Riel on how to improve the Linux VM and what it should borrow from the FreeBSD design. These kinds of exchanges have stopped however. Todays kernel hackers seldom lock at FreeBSD or Solaris (if at all). In some regards, Linux only caught up with FreeBSD with the 2.6 kernel. However, these days, Linux is *much* superior to FreeBSD in most aspects.