Bill, another simple way to think of it is this: Say you're out of shape (like me), and your resting HR is high, and then say you do 8 minutes on a max exercise stress test and your peak HR is 186. Another guy who's been running every day comes in with a HR 52, and peaks at 148 for the same exercise. Let's say they're the same body mass. They've done the same amount of work, but one more efficiently than the other. Yet both exercises were aerobic until the very last minute on the guy that was out of shape. Now you put an art line in, and CO2 sensor--you find out that the both guys had the same VO2MAX, which under aerobic conditions gives you an idea of how much expenditure there is. Same work, same VO2MAX. HR--not an issue, and only reflects the efficiency of the tissue at extracting O2 distally, and the performance of the heart for each stroke. Now put the guy that's in shape on a treadmill for 20 minutes. His VO2 max is higher, he's had more expenditure (caloric)--and the first guy isn't even capable of burning that many calories in one setting. Expenditure is therefore primarily a function of work and not HR, as the flaw with following HR is overestimation of work due to poor efficiency.
Now following METS (metabolic equivalents) on a treadmill for purposes of doing CARDIAC stress testing is different, since we're trying to get a look at cardiac work, not caloric expenditure.