There are some consequences if you do not register for UN 3002. If you are not registered for 2 credit of co-op then you will no longer be a full-time student. This could impact your financial aid (loans or scholarships), visa status, or qualification for health insurance. During the summer, however, most students do not need to maintain full-time status. In this case whether or not to sign up for the co-op class is purely your choice.
For those students here on an F-1 visa, the Career Center recommends that you register for co-op credits during the summer as well. By registering as a co-op student you are considered a full-time student and will not be utilizing your practical training time.