You could say that Trophies and Achievements add a layer of replayability to some games that otherwise might not have it.
Err nope. Maybe soon though as the push to have every game released from now on will spur interest in Trophies.
+1! I never understood why people bagged on Sony for not having it. To me its pointless. It's viagra for your e-peen.
They motivated me to do some things that I may have otherwise not bothered to even try. For example in Burnout, where you have all the billboards to smash through, I would never have had a second thought about ignoring them, but since there was a trophy I thought I'd try. It ended up being awesome because they set them up with some massive jumps that are great when you manage to pull them off!
Bingo I would never have tried playing through games on the extra hard difficulty setting (superhuman, impossible, veteran, crushing etc) however I had a go on a few and the reward is huge at the end (not the trophy, just knowing that I completed it)
Then again some trophies are hair pullers. Meet 8 people in the wildcats? Easy, nope some noobs have always got to go afk or dick about over in Silver Mountain while 7 others are in the stadium.