Whether or not achievements are the "right" motivation to play/keep playing a game, it doesn't change the fact that it is appealing to a lot of people. The weird thing is that Nintendo already understand this, as plenty of their games have unlockables/collectables/achievements already.
One thing no-one seems to have mentioned is that a unified achievement system would help drive 3DS owners towards the Wii U. If they attach something to the achievements, like the hats in TF2, it could become addictive enough to buy a whole system for. Some hats in TF2 go for thousands of dollars. It's pretty much worth putting it in just on the off chance it is a hit.
Also, it promotes loyalty from one generation to the next. I know a few people who are pretty upset at having to leave their gamerscore behind on the 360 when moving to the PS4. If it was a toss up between 2 systems for next-gen, the achievements could swing it.
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