Achievements and trophies make the games worth playing. Games without trophies IMO are not usually worth playing because you get nothing for wasting your time playing them.
Achievements are better IMO because they are original and Microsoft introduced them two years before the PS3 introduced them into games. Every XBox 360 game has achievements. You have to find a list on the PS3 games with trophies. Numerical gamer score looks better than different numbers of coloured trophies.
1000G = 1 platinum trophy. Platinum trophies are annoying because you have to spend hours and hours on that game to get all the trophies in a game before you obtain one Platinum trophy.
It is unlikely that developers would patch trophies into old PS3 games.