Look, as long as all of the modules that get shipped with the kernel are GPL, there is no issue with third parties providing closed source modules.
The module is its own binary. You can use it or not. The kernel developers do not have to use it, test it, or even be aware of it. If it is closed source, it is strictly the vendor's problem to make sure it works right.
The kernel itself should always remain pure GPL. This is goodness. However, do not yield to Stallmanesque fanaticism about all software having to be GPL. If open source is to succeed, it will only succeed by living happily with closed source code and enticing the closed source vendors into opening up, but you cannot get pushy with the vendors or they will take their toys and go home. And no matter what GPL advocates say, there is plenty of really valuable commercial software out there that would make Linux more successful as a platform.