well the theory is that when gas slowly trickles into the dip tube on the liquid side, it bubbles up from the bottom and agitates the whole column of beer on its way up. it doesnt work nearly as well as if you were using a carbonation stone, the bubbles are too big. however the fact that it causes the beer to move around some would probably promote faster carbonation. how much faster than just putting gas in from the top? probably not too much. it might not even be a noticable difference, but the theory makes sense atleast.
also if you are shaking the keg, the longer dip tube on the liquid side puts more of a distance between the beer and your CO2 line, so its harder to get beer to splash up in the line and contaminate it. if you shake the keg while its hooked up to the very short gas dip tube, its possible for beer to splash up into the gas ball/pin lock, and i know i dont clean the gas fittings nearly as much as the liquid ones... its not really possible for beer to splash up 20-something inches of liquid dip tube and reach the gas system while shaking.
all that being said- i just hook my kegs up the normal way and let them carb on their own.