Actually not true, the ribs are very inefficient at cooling, it is the inside flat part that does the majority of it. In my experience a lot of cat riders experience over heating issues a lot of the time. Still a great sled but the flatter new system is more efficient than the ribs, and stronger and easier to fix on the Ski-Doo.
Cat have smaller heat exchangers than Doo, that's why Doos cool better. The ribs do protect the heat exchanger, but of course they can still be damaged. If you are just repairing the heat exchanger and not replacing it then a flat exchanger will be easier to repair than one with ribs, but if you're replacing the exchanger, then a Doo is more complicated only because it's more incorporated into the tunnel than cats. Not bashing just comparing.
Efficient heat exchanger have large surface area to fluid ratios, it's that simple. Increasing the surface area of a heat exchanger, will increase the efficiency of the exchanger. Radiator and air cooled engines use the same principle. Also, a larger amount of fluid a cooling system has, the better it will cool. Ribs increase surface area. Like I said before, I'm not bashing.