I know people well into their 30's who do Xbox gaming nights, 2 consoles 4 people. means at least 2 people need to use online profiles, more if you use 3 or 4 player split screen for 6-8 people.
There are (local) co-op games on the PS3 that allow you to sign into your account with whoever you are playing with to get your trophies/achievements. And yeah save games as well, I regularly play my GF's PS3 when I am at hers and download my saves off the cloud to carry on playing titles - We have our accounts on each others PS3s for downloading games from the accounts as well.
Quite often I cannot be bothered lugging my PS3 to my GFs, so I appreciate how smooth things are on PSN for letting me use my account on her PS3.
It's simply an entirely counter-productive system (one could say overly vicious DRM), and quite damaging to those in specific scenarios like me above.
Okay, 2 valid reasons why this method does suck.
Since I never do either they really did not cross my mind, but no doubt hearing them now makes me realize what a truly horrible idea it was to implement.