I see lots of uses for a fixed IP, one is if you don't run a DHCP server on your network.
@OP: You cannot specify a static IP for the Wireless Shield because all the DHCP stuff is handled by the embedded (on the shield) ATmega168. That source code (the shield's "firmware") is not yet publicly available (to my knowledge), so you're currently bound to the DHCP IP address with this hardware.