It is probably more of a desire to reduce the effects of RSI and prevent jumping all over the keyboard. We don't necessarily write code at break-neck speeds, but we do need to use modifiers and special keys more than the average keyboard jockey.

That beign said... typing speed is also important. I've always written a large amount of prose as part of my job (compared to most non-programmers). And not just time-wasting typing at HN, but documentation, job-related emails, etc.