If the Game Engines are already abstracting this away, why /not/ provide builds for smaller OS's. I know there are extensions to Visual Studio to build binaries for Linux.
I think what a lot of game developers are missing is that gaming is the /only/ reason many users are keeping Windows. I'm 100% positive I know at least a dozen people personally (and I'm not a very social person) that would ditch Windows completely if they could game easier on Linux. Is that extra 1% effort not worth it to you to provide a choice to your users?
I understand that =<1% is minuscule in many developers minds, but if you remember there are roughly 5.6 Billion people in the developed world with access to a computer and the internet. That 1% equates to 56 Million people. Then you must keep in mind those using Linux are (typically) more technically minded and willing to exert more effort to play something natively in their Distro. There are various fans of specific software that literally make it their duty to build wrappers/packages for specific distributions (Package Maintainers) so things work easier for the distribution as a whole. If you provide an accurate dependency list (which would usually be included in the engine you're using), then the distro package maintainers can handle the rest for you.
If your game is well made, and becomes popular, linux users tend to promote companies that work with the open source community. That's free promotion for simply making an effort to appeal to approx 56,000,000 more users.
That's worth the effort in my books... But as you can probably tell, I'm a Linux user.