You don't have to enlist, but there are some advantages. I am going into the Marine Corps as a 92 day reservist. The difference between a 92 day and a regular reservist is that you go to college first and will not complete training or become a full-fledged Marine (deployable) until your Senior year in college. In the mean time, up to your Junior year, you can apply for PLC. This route fills its slots quickly, but again is not required. Talk to your recruiter about it but again, the slots may be filled up already.