All of the following reasons, are the benefits you get from the FreeBSD kernel:
Standardized kernel interfaces:
Single /dev implementation via devfs, instead of the 3 discordant
ways of handling /dev that Linux provides.
OSS as the default sound system (i.e. the standard interface
supported by almost every Unix-like system around).
OpenBSD Packet Filter (pf).
Security features, like jails.
Support for NDIS drivers in the mainline kernel. On Linux,
NdisWrapper is unlikely to make it into the mainline kernel.
Support for ZFS in the mainline kernel. Due to license and patent
issues, ZFS is unlikely to appear on Linux.
kFreeBSD is less vulnerable to legal issues. Licenses are managed in
a centralized manner compared to Linux kernel's bazaar-like
development model.
kFreeBSD developers often have more interest in merging new features
rather than spawning forks all along (the port to Xbox is a very good
example).
kFreeBSD may have better performance and/or stability especially in
disk/filesystem areas with ZFS.
The FreeBSD kernel might support some hardware which Linux does not
support and/or the FreeBSD kernel support might be better (fewer bugs).
Why would you prefer Debian GNU/kFreeBSD to FreeBSD?
if you like the Debian package system (or its package set) more than
FreeBSD ports (just a matter of preference).
If you like GNU userland more than BSDish one (again, just a matter
of preference).
If you don't have anything against GPL or other copylefted free
software licenses, you'll appreciate that useful kernel modules like
ext2fs driver, the upcoming reiserfs and xfs, or the upcoming
ethernet driver for Xbox are (or will be) compiled in on the default
kernel.
If you're concerned about running a 100% free system, our commitment
to the Debian Free Software Guidelines (DFSG) guarantees that Debian
GNU/kFreeBSD doesn't contain any non-free software. In fact, we have
removed some non-free binary-only drivers that are contained in the
upstream FreeBSD tree.
From the official wiki, there you can find more points on why to prefer the Debian GNU/Linux and not be totally FreeBSD.
