The massive hard drive will allow you to "own" HD movies you download off of Sony's new video delivery service. You do have the option of deleting a film after you download it to make space and can download it again at any time for free, but if you don't want to go through that hassle you can just let it sit there. But really, even 80-Gigs is insufficient for more than a couple movies.
There are also quite a few games that load a bunch of stuff on the hard drive when you first run the game, to help with game performance. Some of the installs are 2-5 Gigs. So you can fit 15 games or so without having to delete anything.
Then there's people who load their iPod play lists onto their PS3 for custom in-game soundtracks... If you want 20,000 songs you'll need some space.
And I agree with you... there's never been a point yet in this generation of movie players where it's made sense to buy anything but a PS3 for Blu-ray playback. It's future-proof. It runs better than most stand-alone players. It's about the same price. And heck - you can play games on it if you want!