Oh, thank You. I love to be anyone's favourite (even 'so far'). =}
Harry's reason for wanting to keep the Cloak were in the bit that got cut. He came up with it a long time after the original bargaining (while he was hiding from the household chores in the bath). His reasoning was that: it was his inborn instinct, that prevented him from giving up the cloak. Because since Death lost the Cloak, she couldn't hide from the people any more. That's why people suddenly discovered bacteria, viruses, and other deadly elements and learned to protect themselves from them. 'Putting a stopper in death', so to say.