I see oftne tiems people talking about drilling a drain hole in the BB shell. WHat is your opinion on this? I have an AL frame so its not like it will rust, and what size do you drill it?

Mine all just have a little tiny maybe 1/8" hole. That seems to do the trick. It isn't so important for your frame but it will help to save your bottom bracket.

I have seen others with monstrous holes, but I suspect that's mostly aesthetics.

Water won't hurt a cartridge BB.The monster holes are the work of weight weenies.

Is a drain hole more useful for a cup/cone BB? Is there any drawback (dirt in BB) to having one in the shell of cup/cone set-up? I have the accordion style BB sleeve in there.

Mine have drain holes. The sleeve fights crud.

Water in the frame can lead to your BB being seized in your frame. Major pain in the azz to get out. All of my bikes now have drain holes for this very reason.

The purpose is not to protect the bb cartridge, although that is important. The fact remains there is water coming in. That must be dealt with. Aluminum will be affected by water, just in a slower manner. Water in the frame is bad and must be drained. 1/8" hole is plenty large if you periodically check it.