My concern with the license isn't legal so much as pain of administration. Maybe I'm just old fashion or lazy, but I prefer not to have to check my kernel repo against my kernel module repo to make sure they play nice. It is so much easier when you can just get it as part of the kernel.
That is the reason why I avoid the Nvidia binary driver as well.
If you're on Ubuntu, there's an apt package available in ZFS On Linux project's ppa. It downloads the source, builds it against your installed kernel and installs it. Rebuilds on any kernel update. Very handy.
That is the reason why I avoid the Nvidia binary driver as well.