Splined cranks have a much greater surface to surface contact area on the taper, allowing for a much greater torque force to be put on the axle and crank taper befor failing (rounding) point.
Depending on what spline your talking about,
most bmx 3 piece setups have cranks that slide onto a non tapered axle and then clamp onto the axle, allowing for a very secure fit, with a greater surface to surface contact area. This is the concept that profile hubs use,
Isis spline mountain bike cranks are made with a slight taper allowing for the crank to be pushed onto the spline without a clamp system, still allowing for a tight tolerance fit.
Both these systems rely on the fit of the crank to the axle, and not the force of the bolt pushing the crank onto the taper like a 4 side cotterless. thus releving stress on the bolt and transfereing it to the axle, where the stress should be.
Other points with spline, they use a larger axle, which is automaticly stronger, and has more contact on the crank. Its much harder to round a spline with lots of contact points than it is to round a crank with 4.
Ive broken about 8 sets of cranks, 5 on my trials bike, 3 on my unicycle, before i went to spline with both, they are now both going strong, i have an isis setup on my trials bike, and a profile on my uni!
watch, in a few years, cotterless wont even exist! same as v brakes have taken over from cantilevers!
Hope ive helped,