If you've been following this journal for a while, you can probably guess that I'm in favor of such sites (and wish this sort of thing had been generally available when I was a university student). I think the more practice material you have (especially solved problems), the better. Regarding the time factor for professors creating exams, if the professor is at a school where publication is valued more than teaching, I can see that generating new exams might be a concern, but I still am more sympathetic to the students who can benefit from practice with old exams (who aren't just looking for the answers). Having practice exams also gives the students an idea of what the professors consider important and also a sense of how the exams are structured, so they can better budget their time and so forth.
OCW has all sorts of old exams, bless them. Other schools don't, and in fact, there are rules against redistribution of old exams. I remember reading that in the CS dept of the Technion, it is forbidden for students to give other students old exams. I wonder what students do who need practice test material. Is there a black market for old exams, or is there some alternative form of test practice? Do they just go to other schools' sites and hope the exams they see are representative of what they'll see?