This reason alone is the bane of my RSI issues. I have no problems if I can contain myself to strictly keyboard input. Granted, I will notice some flare-ups if I use heavier switches (clears, panda clears).
Mice, though, cause me daily grief with my neck/shoulder.
Also, multiple monitors are becoming more of a norm, now. Craning our necks to view between multiple 20+" monitors instead of a single screen (or document if we're going back to typewriter days) can keep added stress on your posture and neck.