And so is the traffic by people on cell phones. So is the traffic by blind users. So is the traffic by people using text based command line browsers. So is the traffic by people using feed readers. So is the traffic by... But together it eventually adds up.
And it adds up rather quickly, too. Individually, they may not be very common, but when as a group... (to use an old analogy - a person is intelligent; people on the other hand are stupid)
I've known a few blind people. I also know someone who uses his cell phone as his primary mode of browsing when away from the house, and he is continually frustrated by being unable to use some pages. If I make my pages accessible to him, on his cell phone, he's likely to visit my pages quite often once he realizes he can browse them on his phone.
Dan, you missed the irony of using spam keywords for captcha. All those who read and comment on the blog entries get an instant backlink with PR, none of that "no-follow" crap because I believe in rewarding people who contribute to the content.