Two different issues. The what and the why. You don't need controlled power to see if you are faster in one bike than the other. And I think that there is a lot of value for this "real world" experience.

By comparing speed with controlled power you may find out that bike A is faster than bike B. But if bike A beats the crap out of you when you ride fast, you may very well end up riding consistently slower to avoid being beat up. Or you may find that for a given power output the difference in time on a climb is negligible for two bikes. But in the real world, the lighter bike will encourage you to attack and you will find yourself putting a bigger effort (GBN saw something like that happen on one of their tests. They had a guy ride without a meter display and he pushed himself harder without noticing)
I agree that if you are trying to invest in upgrades finding the weak link is important. But knowing the real world effect is important -and darned interesting too.