I haven't released any commercial games yet, and I don't use Linux myself, but when my RPG project is finished, you can bet I'll support Linux.
Reasons:
1) If you're supporting Mac anyways, it probably means your game structure must be flexible enough to have OS-specific portions. A third platform is less work than the second (It's still work though).
2) Mac and Linux, at an underlying level, supposedly work alot alike. Certainly more alike than Mac and Windows. Mac and Linux also share a few of the same standards among APIs.
3) Linux users, because of the lack of games, are more willing to show support for the games that do make it to them. I personally feel like piracy rates are less on Linux, despite the greater technical know-how. Linux users have very consistently made a point of paying more per user (sometimes averaging double or triple) than Mac and Windows users on the Humble Bundle. Here you have a group of people saying, "We want great games! Here's us spending extra money just to prove it to you developers!", and then you have developers going, "Meh! I'd rather get paid $1 on iOS by people who'll play my game for 5 minutes then forget about it, and who'll never be a repeat customer."
4) The Linux market is less crowded, so you'll stand out more. Linux news sites eager for news will get likely you more coverage than Windows news sites that are flooded with feature requests.
5) A sale is a sale. As an indie, I'll need every sale I can get. If I can port to Linux in less than 90 days, and get at least 600 sales, that would be about breaking even. Anything more than 600 would be on-going profit while I work on my next game (along with any ongoing Mac, Windows, iOS, XBLA, or whatever sales).
6) Once the Linux support code is in your code-base, it'll be easier to port your future games to Linux, meaning it'll take less time, and easier to break even (less sales needed to break even), and quicker to turn a profit for the next Linux port.