Couple answers I'd like to add to some previous questions:
Metal Sting Base (the base from Metal Draciel/Master Dranzer) supposedly had those slits to increase its defense. In theory, they sucked air in and forced it out the top, essentially pushing the blade downward just slightly harder than gravity alone. While this is mostly bunk, there WAS the added benefit of increased stability due to the air flow.
Trygle and Cyber Dragoon both also had another reason to purchase them; their baselocks. Both had large, overhanging baselocks that were phenomenal for breaking an opponents upper attack against you, and in the right height ratio, could assist in a force smash attack from you.
Flash Leopold, UK Superhero, you know where I'm going with this by now...