Here's how it works. You see a title for sale in the store, and think to yourself, I want to rent that. Then you try and rent it and Netflix says it is not available. Repeat experience often enough and you're going to lose customers. Of course, it's only one studio, and the instant streaming library is a nice off-set.
I think you put a little too much weight on the whole instant gratification thing. If it were a big factor in people's movie rental habits, Blockbuster and Hollywood Video would be doing just fine (and Redbox too) and Netflix would be six feet under.
I'm guessing for those who join Netflix, they're sick of paying $5 for one rental and willing to trade a short wait (while the disc is shipped) for practically unlimited rentals at a low price. Most people I know who have Netflix have dozens if not hundreds of discs in their queues, so it's not like they're stuck with nothing while waiting for that new release to come out.