Thanks Auliro, I guess that is what I get for not knowing my "cheese-builds" well enough.
I must have just assumed it was to combat the problems like "minion-pop-charges" which can occur in sensible builds but can take the fun out of things very quickly if it just stops the DM ever using minions as they become a bonus to the Barbarian not the monsters (Assuming the Barbarian has a good chance of hitting per attack).
Remember that Swift Charge is an encounter power. The barbarian can kill one minion and bounce to another one... but then he's done. He doesn't get to clear the whole field.
It's Rampage that he can use throughout the combat - which requires him to crit (harder to control), and is still limited to once per round.