Harpers do not crave for power, political, economical or whatsoever. Do not base your assumptions on what you see in the game. Harpers actually work to prevent the power being concentrated in one place.
That they do. However, Harpers do crave control - and separate that from the desire for power. Good king, bad king, they don't care if the king has a small kingdom. Good king, bad king, they DO if he has a large kingdom, since they call that inherently oppressive. Their goal is to be able to keep polities small. They don't want power per se, but they want to be able to control who has it, who doesn't, and how much they have. And one might ask - is that not a desire for power? Debatable, but it's a sure thing that they want control.
In response to Hunter, yes, they usually wouldn't say such a thing, but Reviane is a Harper and so is Jaheira, and they both understand that truth is not high up on the Harpers' list of priorities.
At least that is amongst their goals - of course there are bound to be bad seeds like Galvarey in any organization. Also, Harpers are pretty much the only 'good' organization that opposes the countless 'evil' organizations like Zhentarim, Cult Of The Dragon, Red Wizards, Rundeen, Twisted Rune... nah, but I guess its too easy to bash them only because you hate Galvarey, Dermin or Elminster.
The churches of the good gods do very well opposing the evil organizations, especially considering how the evil organizations fight amongst themselves. The Harpers don't really care that much about the evil organizations - they do, but only in the sense that they meddle in the Harpers' sphere of influence. Remember that the Harpers' alignment is generally neutral...not good. They weren't written to be universally good - and they aren't.