Ditto on the "you should back up your stuff" message. As people's lives become more and more digital (movies, TV shows, music, photos, etc, etc), people are really going to regret it when their HD smokes and it's all gone.
As far as letting someone else 'hold' content I've purchased: I'm already uneasy enough as it is that so much stuff that I have paid money for and, in some way or another, own exists solely as an intangible download, even if it is on my own hard drive(s). I would much rather have something durable to show for my money spent, like you get with a CD or DVD (or, for that matter, a book!). I know that you can back stuff up to home-burnt CD or DVD, but even that has a shelf life.
What happens if, knock wood, Apple goes out of business or shuts down the ITMS (in some form or another that will happen someday -- nothing lasts forever), and takes all 'your' stuff with it?
A good compromise might be like a lease, where you pay a price that's somewhere between renting and purchasing and you can have unlimited on-demand access. Probably not enough people would go for it to make it worthwhile, though.