For some people, they add to the game once it is complete. Some games use it as a cop-out to add longevity, but some really get creative and make the longevity fun.
Yeah but does it really add to the game if they are system wide instead of in-game. Personally i don't think so.
It's not like the achievements is a new concept. There has been achievement since the day of Sierra games. The points system in Sierra games was an archaic achievements system.
System wide achievements are great but there's 2 sides to the medal. Sometime devs just don't care and add them because they have to and not because they want to. More often than never achievements in 360/Steam games are dumb, easy and/or tedious to get and you feel like they have been slapped in at the last minute because they had to be there.
New achievement unlocked you just killed 10 mobs houuuraayyyyy next one you need to kill 6000 0000 00 0 00 0 0 0 0 mobs ... okay ...
I think Nintendo should add system wide achievements but i don't think this is a deal beraker.
I doubt trophies/achievements will be reset when the next generation is out. They're already cross-platform. Vita/PS3 , W8/WP8/XBOX. The new consoles will just be another platform you can access your account from.
I hope though we will be able to delete them if we want to next gen. One thing i want added to XBox Live is the ability to delete a game from my games list. Sometime i'm borrowing a game for a weekend, complete 2-3 useless and irrevelant achievements like killing 10 mobs or finishing the first level on normal difficulty and then the game is there in my game list forever and i just don't care about it and would love to remove it.
Anyway i never found a way to remove them maybe there's one i don't know (i cancelled my sub 2 years ago maybe it's possible now).