Some people just have really bad typing habits. I've blown out my arm on occasion by not paying attention to how I'm situated. Leaning over to use your mouse, having to reach a bit too much for it, can add up slowly over the course of weeks and wreck you.
If an ergonomic keyboard forces you to keep the proper posture and position, so be it.
And for people who care about RSI -- funny ergonomic keyboards are the wrong answer. Make some exercises. Buy a Powerball; do some push-ups; practice any of the infinite number of Yoga asanas that put weight on your wrists. A few minutes of daily hand/wrist exercise will completely protect you from RSI, no matter how much typing or mousing you do.