Agreed... see Prince's performance at the Super Bowl in Miami.
As for the roof, I thought it was just covering the ring area. I'm pretty certain the field will be uncovered due to the fact they have heavy boarding on it to cover up the grass.
The heavy boarding there is to create a steady enough platform for additional seating, as well as a structural base for the roof and entrance platforms.
The roof is described as covering the ring area, but also being tall enough so as to not obstruct viewing from the bleachers and provide good enough clearance to hang massive video boards, sound equipment and lighting. Considering the steel beams were manufactured in Europe as a specialty order, my gut instinct is that the roof is fairly large.
The article in the Orlando Sentinel indicated that WWE started planning over one year ago for potential weather problems. I don't think rain or a thunderstorm is going to phase them at all.