Hi Turd,
(Makes me laugh every time you post. I love that sketch. I miss the days when people had personalities, and stuff was funny.)
I can't think of a reason not to put insulation there, unless it would somehow backfire and cause more condensation problems. I know the space you're talking about is empty in my Montana and is just part of the rest of the empty space in the whole ceiling, because little mice run around in my ceiling sometimes and end up in the skylight over the shower, where I can see their undersides.