I've started paying attention to my mousing habits since this thread started, and I can say, personally, I'm rarely using both the keyboard and mouse at the same time. And even with a compact keyboard, I tend to move it out of the way when I am mousing a lot, so a lot of the advantages stated here are moot for me.
Besides, I do loves me a nice buckling spring keyboard. Now I just need to work up the courage to do the nut/bolt mod on my M, and then rearrange my keyboard layout.
Too bad I can't find a mod to split a fullsize M into the alpha block and then the arrows/editing/number block. Stupid membrane.