Yes, I think that's how they should be used. Encourage creativity.
Even though there's no guideline for individual achievements, Microsoft state that a large percentage of the 'gamerscore' distribution has to be allocated to things the player will automatically do by playing the so-called 'golden path' (ie the minimum required to simply finish the game.)
This means a large percentage of the achievements have to be meaningless, hence the people who rent easy games just to higher their score. I wouldn't mind if all achievements were creative, but most aren't allowed to be.
I just don't see why we need to layer meta-rewards on top of a game, when the game could provide its own. There's nothing a creative achievement can add that a creative mechanic can't, apart from the public nature. Then again if that's what people like about them, that they are public then I feel that the 'golden path achievements' should be dropped, as should the score.