Even thought the default grub screen _is_ pretty, I see no need to display it until a key is pressed for 30 seconds. 5 secs would be in line with default lilo config, and actually a rather sane value all in all.
I would tend to disagree for it being the default. With the multiple dual boot systems I use, a larger value allows me more flexibility in choosing which OS to load rather than missing the menu and having to wait for the OS to come up and reboot properly. It's an easy enough problem to have when the stuff before GRUB takes 5-60s from power-on depending on the hardware. Besides, you can always choose an option instead of waiting for the timeout or just edit the grub.conf to shorten it. The sort of folks who want a shorter timeout are those that would have the easiest time changing it to be so.
In your scenario, if you have to manually choose something, you are _there_. You should watch the screen. :-) For e.g. servers and all dumb boxes, servers etc. where dual boot isn't required, or the default setting is right, waiting 30 secs by default is unacceptable IMO. ("optimal" solution might be that if you have Linux and non-Linux partitions, the wait be longer, but that might not be "clear" enough and perhaps more complex to implement besides)
Agreed that 30 seconds is a bit long, but I also think 5 is a little on the short side. Changed to be ten seconds as a middle of the road