Helps keep the shoes clean and less glazing up, that's all. It will actually reduce surface area available.
If you race in a rather dirty environment, then by all means, cut a couple of grooves in the shoes...if it's mostly clean and you're not getting oil/contaminants in the clutch, then don't bother.
We used to do that all the time with our oil bath clutches on our two cycles. Hard for me to see how that could have any effect on a dry clutch. Less surface area engaging the drum, I can see a little more slip.