Easiest by what metric? As nknight said, I don't think switching jobs is easy at all. Maybe you mean easier by less social-conflict? But I would think that the "I would like a raise" discussion would be easier than the "I'm leaving" discussion.
It's not easier because your boss has vastly more practice than you do. They're having this discussion with most of the people who work for them. They know how to string you along with carrots off in the sunset that you're never going to reach.
I've seen it done many times. When you set these goals that would be required to get a raise, you're going to want to set definite terms so you can know if you achieved it. Your manager is going to want something more fuzzy. They'll claim this is actually helping you because it gives you more ways to accomplish the goal. In reality, it means they'll always have a way of saying you didn't meet the requirement if they want to (they usually will).
Personally, I never ever work for someone else with the promise of compensation later.
