Split soles are popular because they are popular. Then there are full soles, that come, just like split soles, in suede or hard leather. Hard leather full soles are often referred to as Russian soles; why, I don't know.
Split soles, in my humble opinion, allow a greater flexion of the foot, and can make some feet look good. But, again in my humble opinion, some full leather soles allow greater control of the floor.
I think it comes down to preference. I also think that most dancers need to try many different styles of soles to see what works best in different situations.
balletboy[/i]