A mid plate just hanging out in the breeze, with a couple tabs welded on the chassis is useless, for anything more than holding up the motor or trans when you take the other out. Now, you take that same mid plate and make it an integral part of the chassis, by welding it into the firewall or attaching it to tubing that is part of main chassis structure, you are actually DOING something to stiffen the car.

The pic is a stock firewall car. You sink a piece of tubing in the firewall between snout bars and tie mid-plate in like this and the car will be really ridged in the middle.......that's good