The problem with making an extrinsic reward the only destination that matters is that some people will take the quickest route there, even if it means taking the low road.
Indeed, most of the scandals and misbehaviors that have seemed endemic to modern life involve shortcuts. Executives game their quarterly earnings so they can game their performance bonus. Secondary school counselors doctor student transcripts so their seniors can get into college. Athletes inject themselves with steroids to post better numbers and trigger lucrative performance bonuses.
Contrast that approach with the behavior triggered by intrinsic motivation. When the reward is the activity itself—deepening learning, delighting customers, doing one's best—there are no shortcuts. The only route to the destination is the high road. In some sense, it's impossible to act unethically because the person who's disadvantaged isn't a competitor but yourself.
I've written about Daniel Pink's work on motivation on this blog before. I read Drive a few months ago, but have found myself thinking about it ever since.
What strikes me most about it now, is the similarity between Dan Pink's description of intrinsic motivation and W. Timothy Gallwey's description of the value in winning. In both cases, the external reward—whether it be money or winning a tennis match—recedes into the background. The important thing becomes a much longer term goal: continually improving at something you care about. The effort expended is hard but hardly feels like effort.
Working for an external reward only gets you so far; working to improve—and believing that you can improve—will take you as far as you're willing to go.