Growing up on a farm, and butchering our own stock, that's the way it was done. Although the initial shot was directly to the front of the skull, you always cut the juggler vein next, to bleed out while the heart was still pumping. I can see how this carried over to game animals, and became routine, but it isn't necessary 99.9% of the time with game animals due to the fact as others have said. "It isn't necessary if the heart isn't pumping".
This is what we always did with cattle and pigs. Small caliber to the skull - slit the throat to get them to bleed out while heart is still pumping. It only takes once where someone uses a rimfire and ricochet's a bullet off the forehaed portion of the skull and knocks them down to realize that slitting the throat will either kill them while waiting or wake them up for a whole new form of entertainment. But I believe this is where much of the tradition comes from - at least from what i have seen.
worked just fine for me at 310yds on a mulie last year i have mine on 325wsm. 
Got a bunch of elk in the pasture on and off all week but not a legal bull in the bunch. I suspect that the shooting that my wife heard the other night was the demise of the 5 point herd bull. 
