My guess is to make the single fan more effective. since the shroud covers the whole radiator, having an open hole would allow the single fan to suck air from the back of the radiator (through the open hole) instead of through the radiator itself.

Obviously the best solution is to put both fans on.

I forgot to measure my Trans Installâ„¢ tool. Will try to remember tonight. It really does make things as easy as Darren said.