If anybody else knows the college-wide ID or has it, they can't do too much. They could possibly log on to the system if they know your PIN and drop and add a bunch of courses, but they couldn't create a credit card.
There's no other reason behind it whatsoever. Anything that the offices will be putting on the computers that will be going out campus-wide should now, for the most part, be having the eight-digit college-wide ID instead of the actual social on there.