Yes, I understand all your points. BUT Why not give the user the option to choose?
Why force them to be stuck in a situation where they have to frustratingly try to keep the screen hinges from closing too far. I HATE this. I want to be able to put my Macbook undermy bed over nightand rip/encode one of my DVDs to h264 for the next day's subway ride... I don't want to have to prop it open just to accomplish that.
As to the person who thought you had to select sleep from a menu on a PC laptop, That's not the case, often the manufacturer would include a Sleep button on the case, or you would hit a combination of the "fn" key and one of the F-keys, on my thinkpad it was FN-F4... pretty damn simplle.
On the other hand, If I had to sacrifice the quick sleep option of closing the lid, to gain the extra feature of my computer keep chugging away, that would be phenomenal.
As for the person talking about Heat, The Macbooks don't use the keyboard to expell heat, it's simply an intake for the rear vents. This intake of air can still be accomplished while the screen is closed.