That's what you say. But do you have any prove that a factory reset is really necessary after every update? I doubt it. It's just your standard answer. And if you read different mobile phone forums, most of them say a factory reset is absolutely NOT necessary. Also most of the people here who had a problem and did a factory reset after that still had the same problem.
It reminds me of the people who recommend you to wipe your Windows installation and do a "reset" no matter what problems you have. Which also is total bullshit. If something in my house is not working I also do not break it down and build a new one.
No offend, it's just my experience.
I respect your experience, but i can tell that a factory reset solved many problems. One of them, that i remember clearly, was about the battery drain issue still present after the update wich was released to solve it.I know what i say.
Have you ever gave a phone to a repair center? the first thing the do, even if they have to replace the LCD display, is to reset your device to the factory state. Guess why