i installed ATK before i read up on how the OS handles programs.. during my own daily use and unofficial testing, i agree with how the OS manages things and have come to my personal conclusion that it is not needed for the EVO i use.
for example, if i dont go to a certain sense screen after some time, then i do swipe over to said screen, it takes a few seconds to load the program (the friend stream app for example). then if i go back to home and then , say in a few minutes access the same sense screen, in this example friend stream, the program shows up immediately and refreshes at the same time..
even apps that are not on the today screen... i mean on the sense screen... but are in the apps list, some of those that i have not accessed in a while take a few seconds to initialize... those that i use semi frequently, take less time to load than the ones i rarely use.
also if i back out of a program if it has no "X" or "close" command in it, this does seem to unload the program from memory rather than just pressing the home screen or "tap and hold" to bring up other programs... doing the latter would keep the program in memory.
this is my experience. i do not use any task killers.
my phone is stock all the way with the latest htc update
i have all radios on 24/7 and have the nexus live wallpaper in the background.
i have Gtalk running 24/7 youtube app, a twitter program, fring and some other apps that are on sense.. the evo i use shows no lag.
of course YMMV but this is indeed my experience.