1. If you like the Debian package system (or its package set) more than FreeBSD ports (just a matter of preference).
2. If you like GNU userland more than BSDish one (again, just a matter of preference).
3. 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.
4. 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 (see 903_disable_non-free_drivers.diff in kernel sources for a list).
Of which 1, 2 are just "you prefer it", 3 you can compile those drivers into FreeBSD if you want them. 4 is reasonable, but overall the reasons to use it over the better supported FreeBSD are weak.
The Gnu tools are of course portable, so you can install them from ports - you will need them for compiling software that assumes Gnu options for example.
Debian/kFreeBSD is a weird idea for me, you will have no support either form the FreeBSD community or the Debian community. I would go straight for FreeBSD proper, it is very well documented.