Also heart rate numbers in a fit athlete remain stagnant where power is always changing.

For example, I may choose to complete a typical "tempo" ride (something I do all year round) with 170 bpm average over 2hrs. In late fall when I am just starting to train for the following year, when I have been lazy for a couple months, I may only be able to produce 220 watts over said ride at 170bpm. In the middle of the summer, when I'm hauling ass, I can probably hit over 250watts.

That power differential (30 watts) is huge when 10watts may be all it takes for your to be able to hang on in a race situation, or get dropped.

/end nerd rant