Sorry, i had my numbers backwards.
But, here is the deal with antifreeze . . . as most of you know (and as Rogue said) water is the best for transfer of heat as it is "wetter" than the antifreeze. the primary purpose of the antifreeze is to increase the boil-over and freeze points. the 70/30 mixture gives you the best boil over and freeze capabilities (around 276f and -64f), BUT it does not give you the best heat transfer. the glycol does this by contaminating the water. it is much the same as if you try to heat plain water compared to salt water. the salt water because of the "contaminate" salt, will boil at a higher point and freeze at a lower degree. The second thing that antifreeze does for us is to aid in keeping the coolant systemfree of rust. if you ran pure water your engine would rust (of course the aluminum wouldn't but you get the idea). the cleaners in the antifreeze help keep unwanted contaminates off the radiator which aids in heat transfer.
With all of that said, why do they recommend 50/50? well, it gives you what the industry considers the best of both heat transfer and corrosion resisitance. by moving your percentages from there you can add more corrosion and boil over protection by increasing the antifreeze or you can increase the heat transfer by adding a higer percentage of the water.