There is a direct benefit for us to having a higher Linux market share. Hardware and software companies have a more incentive to create products for Linux. This translates to more drivers available to Linux, for example, or more commercial grade software available to us.
On a more idealistic level, desktop OS is dominated by Microsoft. This is not completely bad, because it allows a common standard for developers. Instead of trying to make their software work for a hundreds of OS' they can reach a majority of users on just Windows. In the end though, its a monopoly, and this harms users. It would be better for Linux (or some other free OS) to become the standard.