The system would kill something if it needed the memory. Just because you free memory it doesn't mean the OS is going to suddenly start using it. If you really get a measurable difference its most likely because you killed a bad app not that you have more memory.
It's the same in every thread or story about app killers. Even though they are tell us free memory is wasted memory and that killing stuff is bad there is always some one that says its all BS because they're phone starts working better once they kill task. Its not the freeing of memory its becasue they have a bad app and instead of finding it and getting rid of it they kill all kinds of stuff in order to kill one bad app.
Now I know this isn't an app killer per say but your goal is the same, killing apps to free up memory instead of figuring out what the problem really is.
Call me silly but I believe the makers of the OS which knows far more than us and knows how the insides of the phones work. And when they tell us this is not a windows phone and doesn't use memory like a windows pc and that freeing memory doesn't gain you anything and in fact can hurt I tend to believe them.
But like I said to each his own.
PS I have no auto killer or app killer and I only have 50 megs free (which seems to be the system default) and both my phones are very fast. Well after I ditch those crappy launcher and skins that is.