In Hourglass they are very useful for keeping the mrl's off the hill. With the type of missles they use it's possible to curve them right over the top (although most people don't seem to know how to do this).
I've pushed back countless mammoths by curving missles over the hill, since they couldn't get down to shoot me because of the obelisk 
EDIT: On the flip side though...
I've taken down waaaay more stealths on Islands and Complex with a PIC sydney, because they always try and get away and can only take 5-6 shots =)[ April 25, 2002: Message edited by: Accurain ]