Just got me thinking and I decided to publish my thoughts.
Two main reasons:
1) I do not like lock-in - being forced to stick with one company's products and services.
2) I value my freedom of choice - I do not like my computing experience being directed by some company's vision and marketing goals.
For me to use Linux, I needed to have these conditions satisfied for my three main uses for a computer : Internet, Office & Gaming
First, Microsoft controlled the way I browsed the Internet by forcing me to use Internet Explorer - sites that did not work in Internet Explorer were a pain to navigate.Firefox helped me out of that hole - I could finally browse the Internet and expect the same experience on any operating system Firefox would run on. Google sweetened the deal because I now had a search engine that was not controlled by Microsoft giving me search results that were really useful. Google has gone on to give me more and more reason use it. However, the moment Google becomes another Microsoft is the day I stop using it. Anyway, I was now free as far as browsing the Internet went.
Microsoft still forced me to use Windows - because I had to use MS Office for stuff like resumes, spreadsheets, presentations and the like. OpenOffice helped me out there. I was no longer tied to Windows on the Office front, because OpenOffice worked on Linux as well. Those same documents would open without formatting changes and I could easily adapt my limited MS Office skills to work with OpenOffice - the cost to benefit ratio made it worth it. I was now free on the Office front.
I have always been a gamer - a casual one. Gaming helps me relax and is fairly important to me - I designed my computer so it could cope with the demands of the latest games. However, at this point in time, I am still forced to use Windows to play any game I want to. I hate being in this position and am therefore forced to keep Windows around for when I want to play a game. I do not have the skills to help change the way games work on Linux, but I do have some money and I will try to help the cause the only way I can : by buying video cards from whichever manufacturer gives me drivers that work well with Linux (NVIDIA: I'm looking in your direction - your drivers messed up KDE for a lot of people) and only buying games that also work on Linux. I do not have a lot of money to throw around and I am a statistic, but when multiplied enough, any statistic becomes significant - If enough of us do this, it will eventually change the market and force game companies to take notice and start releasing games that work on Linux as well.
At this point, I must mention id software, a company that has been releasing games for Linux. The game I'm currently playing (and really enjoying) is Quake Wars on Linux. In fact, I'm enjoying it so much that I hardly play any other game. So thank you id, for letting us gamers play on Linux. I will be buying more of your games in the future.
So that's why I'm now using Linux, in a very big nutshell. There are a lot more reasons, but these are the ones that matter the most to me. If you are a developer involved in an open source project, I thank you for helping to make my time at the computer so much more enjoyable.