I also do not ride my bikes in the dirt, the rain or even through puddles. After almost every ride I find quite disturbing quantities of filth on the bottom of the bottom bracket. This road dirt is often very gritty. I wouldn't want any of it mixing with my meticulously greased balls. Anyone who rides a bike with a cutout under the bb has gritty balls, and needs a sleeve of some sort. I'm with Sheldon that even if there is no cutout, there might be tube filth mixing with your clean grease in the bottom bracket.
Garth Libre in Miami Fl USA