I can't imagine cutting leather in a serious fashion with anything but a round or head knife. Most items I cut out need to be skived on one end and I already have the knife in my hand to do that job with, I don't need to pick up another tool to skive anything with. I can straighten a side of leather after marking a line and sticking to the line and get a straight cut every time. You must have a sharp knife though, to do the job correctly, that is all.
Tony.