A rather long time ago, a commentator on my entry on the practicalities of non-GPL'd kernel modules wrote, in part:

ZFS is most beneficial to companies running storage servers. These companies will only use supported modules [...]

I actually think that this is backwards. Ignoring the various legal issues for the moment, companies building commercial storage servers are exactly the people that I would not be surprised to see using a kernel module that was not and could never be in the mainline Linux kernel.

(I agree that organizations rolling their own hand-built storage servers on commodity Linux distributions would be crazy to adopt any out of kernel module that was not really crucial and very well maintained, or at least trying very hard to get into the mainline and likely to succeed.)

More generally, I think that any company selling a black box system based on Linux has a number of decent reasons to consider using out of kernel modules:

• delivering a black box instead of a Linux system (or distribution) drastically reduces the pressure of updating to new Linux kernels and thus the support burden of dealing with porting code and so on.

• they may well really need the features that the out of kernel module gives them, or to put it another way such modules can give them significant competitive advantages (as in the case of ZFS).

• if they're doing a good job, they already need to have fairly deep kernel expertise on hand in order to tune things for their specific environment and to fix any bugs that you run into.

The latter deserves a bit of elaboration. First, the attraction of black boxes to your customers is that they just work. Thus, as a company you do not have the time to wait for anyone else to solve the problems you run into, either before you ship or after you ship; no matter what, you need your own local expertise so you can fix problems in a timely manner.

In a sense, the extra work required to make out of kernel modules work is a competitive advantage. In the long term, a good part of your profits are determined by how much value you add; the more value you add, the more profits you get (conversely, the less value you add the less money you make). Doing the work costs you money, but if you chose correctly it gives you more than that back in extra value added.