i think your main problem would be finding a squad who would let you participate at the "minimum level" most squads have a probation period before you can drive a rig, or even be considered a full member to ride on calls.. regardless of how long you have been an EMT. if the local squad will take you that is great. also.. you should look into all of the neighboring school towns. if the local towns are paid (or you can't drive there in a resonable time to answer calls), maybe look within a 15 minute range and see if there are any squads that ride overnights where you stay at the building for a 12-hr shift. and.. the final reason you should join is that to recert you are going to need a bunch of ceu's and core 13... core 13 alone costs around $300 but will be covered by the state if you are an active member. hope this helps.
You'll probably ride as an extra person for a specified amount of time. The good news is since you won't have time to take a driving course you'll get to see a lot and participate a lot in patient care. In my county (Orange, NC) volunteers must ride a minimum of 12 shifts before being "cut loose", but i'm seeing more precepting than i probably will afterwords. Short answer, I'm suprised they still want you since you won't be much good to them, but you'll get to see a whole lot of cool stuff, so i'd definitely do it.
If you are doing it for resume padding, don't. If you are doing it because you want to and are interested, then do it. It's a waste of time for the former...being an EMT is not the end-all and be-all of getting accepted. If you are interested, you'll get some valuable patient contact and the most minimum of medical knowledge (like close to none, but better than nothing).
Last summer, I applied to AMR in my hometown two months before school ended so they decided they wanted me before I finished school. Too late for that. (They were pretty desperate...they even called me and tried to get me to start while I was still on the other side of the country). I also worked for my schools student run student staffed ambulance. What you get is some small step up compared to other students when you are in school (but from what I've heard, they catch up fast). How much it helps your application is debatable.