Splined BB's are supposed to provide more surface for your cranks to grip. I have one on my touring bike and I really really hate it. but that's also because it's a cartridge BB. Shimano's octalink is completely proprietary and anyone wanting to make compatible parts has to pay Shimano. ISIS is like Linux in that it's a free basis to build off of. no patent there, or at least a public one if any.
p.s.- I've just spent a week listening to pro mechanics ***** about how Shimano is making everything proprietary in order to make you buy new parts and more expensive ones. Run away from Shimano while you still can.