This is a classic that may help you understand better why some people choose BSDs: http://www.over-yonder.net/~fullermd/rants/bsd4linux/01

Also, there are still new things being invented for BSDs (and Illumos) that aren't available in Linux. E.g. FreeBSD has Jails and Illumos has Zones which provide a complete solution for "containers" which are put together from different components in Linux which overall may not be as secure as Illumos Zones. Another one is the PF firewall. In OpenBSD they introduced security features like pledge and unveil.

Another thing is ZFS:

- on Linux there are legal problems with shipping it out of the box;

- it's far more popular on FreeBSD and Illumos than on Linux, so it wouldn't surprise me if there are bugs that will show up on Linux, but not on the other two.