This may seems odd but, in a previous life I raced motocross around the southern US. The discs that has holes or slots are actually for DRY conditions, and the solid discs are for mud use. Odd as it sounds the holes allow build up of material (mud) between the pads as the solid disc is "supposed" to keep it out. If you watch AMA Pro racing on TV and they have a mud race you will see the majority of riders with solid discs for the mud races. I find it odd that Team Honda was one of the first to use these, and they don't put them on there Utility ATVs.