With all due respect to the OP, your phone is heavily modified(arguably). While this thread has provided great discussion for inquiring minds, I do think your opinion on this matter carries less weight due to this. It is kind of like saying your truck gets 5mpg better than everyone else's and then we find out you have a different ECU, airbox, etc. and that you use a different fuel blend not readily available to others.
Personally, I use the "judicious" method using ATK. I have been an Android user for almost a year. There are some apps that do have "memory leaks". There are apps that "runaway". It is for those apps that I like to use ATK. I use my phone ALOT(sorry, I couldn't resist) in airplane mode. If you have an app with a memory leak, you will definitely notice it in airplane mode. Nothing like watching your battery drain before your very eyes. If you put your phone in airplane mode and then kill all apps, the battery will last about a month....without killing tasks you will last about a day or two. So to say idle apps don't use battery I think is wrong. I know, IN THEORY, they do not. In reality, they do. One of my favorite games(Air Control) does not "sleep" properly when "exit"ed. It must be killed. This used to be VERY apparent on the Samsung Moment when it would cause the screen to wake up about every 10 seconds unless I killed it.
Anyway, if you really wanted to do a test...put twp identical phones used identically and then put them in airplane mode. On one, just leave it as is. On the other, kill all apps. I guarandamntee you, the one with the killed apps will last SIGNIFICANTLY longer. In theory, it should not. In reality it will.
I think this is a great example of "chaos theory". A very short explanation is the first few sentences from Wikipedia:
Chaos theory is a field of study in mathematics, physics, economics and philosophy studying the behavior of dynamical systems that are highly sensitive to initial conditions. This sensitivity is popularly referred to as the butterfly effect. Small differences in initial conditions (such as those due to rounding errors in numerical computation) yield widely diverging outcomes for chaotic systems, rendering long-term prediction impossible in general.[1] This happens even though these systems are deterministic, meaning that their future behaviour is fully determined by their initial conditions, with no random elements involved.[2]
IOW, the "builders" of the android OS have no clue how developers will develop apps and therefore cannot determine the final outcomes as to how the app will affect the OS.
So, in theory, your initial posts and subsequent follow up posts are true(modified phone notwithstanding). In reality, I will use ATK due to the unpredictable potential chaotic nature of how apps interact with each other and the OS.