Why should I use Debian 6 with FreeBSD kernel?

I think the most compelling reason would be to run ZFS under a familiar GNU/Linux userspace.

Debian kFreeBSD is officially considered a technical preview right now. This means it works but is not completely ready for production use.

If you just want a usable system stick with Debian Linux for now.

Once it graduates from technical preview status, you may want to reexamine it if you have needs that are better fulfilled by BSD than Linux, such as ZFS and the OpenBSD Packet Filter (pf).

Debian does not target a specific kernel. Debian GNU/Linux is just one variant (the most popular and advanced). There are also Debian GNU/NetBSD, Debian GNU/Hurd, Debian GNU/Darwin, and as you mentioned Debian GNU/kFreeBSD (and perhaps more). Porting Debian to non-Linux kernels is useful for people (users, system administrators, system developers, etc) who are using/developing a non-Linux kernel but would like to take advantage of the Debian (dpkg, apt, aptitude, debconf, the policy) and GNU (coreutils, autotools, bash, gcc, gdb, etc) tools.