Too expensive, for one. Too complicated for another.
I mean, given this technology, you'd probably have to focus too hard on using your computer, thinking your mouse this way and that as well as whatever actions your guy in a game would have to do. What happens if you have a fleeting thought of the oven still being on or something? Does your guy kill himself? Does MS word shut down without saving your documents?
Even if it works flawlessly, you won't be able to think about other things while using your computer. You might not even be able to watch TV and use the computer at the same time, as I'm doing now, beacuse it might make you type your thoughts about the show that's on at the moment, or it might even interpret the audio signals your brain is comprehending and turn them into text.
Not to mention the possibility, even probability, of hackers developing a tool to "read your mind" as you use the computer. Seriously, how many times a day do you think to yourself the password to something as you type it?
While it seems futuristic and interesting, it's rather impractical. Especially at this point. Maybe in the next century or so.