I agree with those that say that an exo will help keep your rig from being heavily messed up in a rollover. Unless you roll over in a smooth parking lot, you should still excpect some cosmetic sheetmetal damage. I also agree with those that say that an internal cage, or at least some type of structure to tie the exo into is needed.
I built a full interior cage from the B-pillar back to keep the driver's area open. The exo ties into the internal through the roof, and then the roof sheetmetal welds to it to seal it back up. I still have a few pieces to add to the exo to finish it up.
This hybrid cage was put to the test earlier this year when it went over hard on a steep rock face. No glass was broken, and everything stayed where it was supposed to. I had a few dents to pop out, but that was it. The roof defiately would have crushed in like a soda can without it.
(These pics are from back when I was still building it.)