If Linux is going to make significant strides in increasing market desktop share, it needs to be able to compete with Windows in all areas. The average computer user typically just wants a single operating system to use for their work, to surf the net, to keep in contact with family and friends, and to have some fun. Having to reboot between operating systems is frankly too inconvenient for many users, as well as being perceived as too difficult. Whilst you can run many Windows software from Linux (e.g. by using virtualisation software, or Wine), again this simply appears to add to the complexity of using the PC. Furthermore, whilst Wine does allow some of the hottest Windows games to run under Linux, it is inevitable there will always be some incompatibility issues.
Even though PCs face increasing competition from dedicated gaming consoles, PC gaming will never die. If Linux is going to dominate the desktop market, it needs a good stream of native games. However, commercial gaming companies are only going to port games to Linux if they can realise a tidy profit from that work, and in the main that requires a significant gaming base. In many ways, open source games represent a solution to the Catch 22 situation, making Linux more attractive from a gamer's perspective, and increasing the possibility of more commercial games being released on the market.