Most of you are asking the wrong question. It's not why FreeBSD needs Debian, it's other way around.
As you can see on their wiki, there a lot of sane reasons to prefer Debian/BSD over Debian/Linux, but reasons to prefer Debian/BSD over FreeBSD are purely subjective - if you like apt more than ports/packages, if you prefer GNU version of UNIX userland tools over BSD ones, if you want your system to be blob-free, etc.
I can clearly see why Debian folks want this project - FreeBSD kernel has distinct advantages over Linux and that's nicely listed, ZFS, devfs, overall lighter and cleaner kernel, no initrd bullshit, OSS, etc...my former company had a lot of solutions developed around Debian Linux, and there were some problems regarding the kernel, backward compatibility etc. I never managed to push something on FreeBSD because other guys were strict Linux guys; however they don't care for the kernel itself, they want to use familiar userland with familiar hierarchy. This project, when it gets to release phase, could replace Debian Linux installations.
So, Debian administrators that want superior storage mechanisms or any other advantage that FreeBSD provides over Linux on kernel level would simply switch to this distribution.
This project won't affect FreeBSD itself, it's not a competition to FreeBSD or a way to get FreeBSD with easy tools. It's simply an option for Debian users. On the other hand, we get more eyes and hands on FreeBSD kernel tree, and something could be backported in future.
To conclude, i don't see any reason to dislike this project, be it from angle of Debian/Linux or FreeBSD user. However, the name is quite muddy, and without investigating further, one doesn't know what he gets from the name itself. A lot of people think they will automatically get Linux drivers on FreeBSD. Biggest computer magazine in Croatia had a small article on their website about "Linux getting ZFS support". Of course, that confusion came from incompetent article writer that didn't investigate what Debian/kFreeBSD really was. Naturally i wrote some comments there, in really harsh tone, but it will steer out the confusion and misinformation about "Linux getting ZFS".