In a close game, it makes sense to take the 3 points and make sure you don't come up empty (depending on the amount of time left in the game, of course). In a complete blowout, it makes more sense to just go for it since the consequences of a turnover on downs are so much less. If nothing else it's good practice for the offense to build confidence that they can convert on 4th down. I don't see anything wrong with kicking the crap out of an opponent and rubbing their collective nose in it.