Agreed that the calories burned is not the same as the true number. But as far as useless, I think it depends what you want to do with the number.

I have very successfully used the calories burned from a Polar HRM (so it uses heart rate, age, gender & weight) to track calories to lose weight. What I did was set the weight in the HRM to 25 lbs below my actual weight. As I got closer to my target, I made it 15 lbs below. I would also adjust the final number downward based on the type of ride (or hike or swim or run) I had done. If it was a long easy ride with slower friends plus a couple of stops I would adjust downward about 20%. If it was a hard ride with attacking hills and pushing down the other side I would adjust downward about 10%.

Over the course of several weeks and months it didn't matter whether any single recording was correct. What mattered is that it gave me usable estimates to adjust my diet accordingly. I lost 40lbs with the HRM's calorie count being one of the inputs I used. (That I am now once again trying to drop 20-25 lbs is testament to what happened when I stopped tracking everything, but that story is for a different thread.)