Judging by what is written in the novel, I'd rather say that Starkiller is a master of lightsaber combat.
The real reason is obviously because the devs realized how very cool it looks.
If you really want to imagine the character himself had a good reason for it, then it'd probably be because it is a combat style unlike that of any other force user.
That makes it a real pain to fight against for other truly skilled combatants, since they can't predict what'll come next as easily as they could with a more regular combat style.
Oh, did I mention it looks cool? ;D
As for decapacitation...I'd definitely appreciate it being in there (the novel even cared to describe how blood changed the helmet on a rolling head at one part), but it's not a big loss if it isn't.
If I'm dying to cut off limbs, Jedi Outcast/Academy is my release.