There's a large difference in feel between 30 Hz and 60 Hz. So much so that some devs will be absolutely inflexible about dropping below 60.
Also, it's very important to people's perceptions of performance (whoa) to maintain a consistent frame-rate. The variations are much more noticeable than the absolute rate. Finally, on lots of hardware, you can't actually display at 59Hz, say. You'll either be at 60 or 30, and will oscillate between the two in a most annoying way.
