to be honest its not there to let you share your games with other people, it is so you can use your purchases on your own machines. The whole 'game-sharing' thing was people abusing (or using, I'm not one to judge) what was actually a fairly open way of d/ling content. You can't do this with Steam or XBLA (to the best of my knowledge). But really since they changed the license to only 2 machines, its kinda become more pain than its worth, I'm sure a few people with 3 ps3s in the house got butthurt by it, but I think if you've got 3 ps3s in your house you can probably afford to buy a game twice. A few games even expect you to be signed in on the psn you bought them with.

Its a bit of a messy thing really, publishers expect a game to work on only one thing at once, if you bought a disc based game, you can't play the game while your friend has it (unless its Portal 2). But downloads are not physical products, you can just make multiple copies. They see it as stealing, in the same way they see used games as stealing profits from them. If they are right or not is quite the debate...