In short, "Do this and you'll get that" makes people focus on the "that" not the "this." Do rewards motivate people? Absolutely. They motivate people to get rewards.
It's important to distinguish well-conducted from poorly conducted research, and to understand the outcome variables in a given investigation. For example, if someone were to announce that studies have shown traditional classroom discipline techniques are 'effective,' our immediate response should be to ask, 'Effective at what? Promoting meaningful learning? Concern for others? Or merely eliciting short-term obedience?' Empirical findings can come from rigorously conducted scientific studies but still be of limited value; everything depends on the objectives that informed the research.
Very few things are as dangerous as a bunch of incentive-driven individuals trying to play it safe.