Tripwire: "About 10% of our game sales are now on Linux and Mac OS"

[Zeven]

What weve seen in the past since Steam has come to Mac and Linux is that about 10 percent of our game sales are now on those platforms. That extra 10% in sales was definitely worth putting our games on those platforms.

The above is only an excerpt. Here is the source.

This shows that being a big fish in a small pond counts for a lot. Despite Linux and Mac OS combined only accounting for about 5% of Steam users, there is a huge amount of pent up demand for games on those platforms. Valve's push will only aid this further.