Your argument basically calls for casual users to stop being casual and to begin to become experts. Activity monitor certainly does help casual users understand what's going on. For the casual user using a system normally (meaning having a browser open, checking mail, writing in a word processor) seeing that application X is using the most memory means that's the problem to them and they're correct to assume they should kill it. It may be hit or miss but that's all they know and in most cases things get fixed that way.
As professionals we often forget what it is to be a casual user. Asking a casual user to learn what you explained as folklore is simply too much to ask. Everyone who uses a computer should at least be technically literate to a degree but that means understanding the basics. To a casual user the basics are: my computer has a processor that executes tasks, it has RAM that stores data for quick retrieval, and a hard disk for long term storage. Each application uses a percentage of my finite RAM and when it runs out my system slows down. Therefore logic dictates that if I kill the app taking the up the most RAM my computer will go faster.
That's all they usually know. We understand that Activity Monitor lies to us and killing random processes is voodoo but we also have to take into account how we use our machines. The casual user will be able to solve their problem by killing processes more often than people like us will because of the way they use their systems plus there is a placebo effect for them. When they kill a process they often feel like the system just got faster regardless of whether it really did.
I liken it to driving a car. Ask some random person about fuel economy. Their thinking is "high octane fuel has more energy per gallon therefore if I use it I'll get better fuel economy". They might even know the relationship between tire inflation and fuel economy too if you're lucky. Ask a professional driver about those things and they'll look down at the average person like they're crazy. They know all how octane, oil, air filtration, shocks, struts, aerodynamics, etc, etc. all contribute to better fuel economy. "If only the average driver knew what I knew, then they'd save a ton on gas" theyd think. But alas, that's too much to expect so we just have to make sure they get the basics and it's up to the professionals to provide the average person with something that just works and do our best to be one step ahead of users by anticipating their usage patterns. This applies to hardware/software engineering, car manufacturing, and anything else. You just can't expect the user to learn or even take an interest in even a quarter of what we know.