I'm just a cheapass who doesn't understand who people are willing to use an arduino to do the same thing that you can do with a $5 microcontroller. Don't take this the wrong way. I'm really very grateful for the contribution. This device will allow me and many other members to finally use all of those awesome terminal and XT keyboards we have sitting around.
Ah, well, I can explain that quite simply: it's easier!
I chose Atmel AVR mainly because there is a good, free, C compiler. But then their range of devices is frustrating - hardware USB support is only available in a surface mount package, so some kind of breakout board is needed to make it usable by most people (the chip itself only costs a few dollars). I haven't tried V-USB, but as it's a software USB solution it uses a considerable amount of CPU time, and I would be concerned that it would not be easy to acheive the same level of reliability in reading data from the keyboard as I have using the hardware USB (which mostly just does its stuff in the background). Even with the hardware USB, I've had to solve a few tricky timing problems.
It's easier for the end-user as well. Hardware USB means you can load code into the device via USB (and as a bonus, the Teensy loader is well nigh foolproof). Without that, you'd need some other hardware to load the software onto the chip, which most people don't have. At a minimum only four wires need soldering, and that's a benefit too.
In essence, a Teensy does give us useful extra features compared to a $5 micro on its own.
Were I to take offence, it would at your use of the word 'willing', possibly implying that I've been had over by some evil scam to part me of my money! Or that I am a slack engineer who is wasteful! I would of course deny both, having considered V-USB and PIC options, and deciding that the Teensy is the best compromise between economic, efficiency, and ease-of-use factors in order to get the job done :-)