3rd Reason – Specialty Hardware
Sure the open source community is great at developing drivers for a variety of obscure hardware but manufacturer support is definitely appreciated.
You probably all remember Linus Torvalds with his somewhat recent and brief NVIDIA rant. In defense of NVIDIA the company thankfully takes the Linux platform seriously enough albeit with proprietary drivers. NVIDIA is also decent enough about porting new features over to a variety of platforms, open and closed.
Some hardware support has not been quite so fortunate – I draw your attention to dial-up modems.
Thankfully the need for 56K modems has all but evaporated for the most part. If you happen to have a laptop with a 56K modem embedded then pretty much count on not using it under Linux. There has been efforts to create support under Linux and BSD but the battle has been uphill and it never was user friendly.
You could find serial-based modems and more recent USB solutions which are Linux compatible. Still, embedded 56K modems embody the problem with Linux support from hardware manufacturers. At the very least – if you make hardware you should give the community assistance with supporting themselves.
What they can gain:
This one is pretty obvious – you can sell more hardware if you have more potential customers. If you ignore a market they will ignore you. Even if the difference between whatever resources you decide to allocate and your increase revenue is a dollar: that dollar is one which you willingly left on the table. If someone else takes it you only have yourself to blame.