Although I agree with there not being a need to always be closing apps to free memory (any more than there is a need to trim the registry in Windows Vista / 7 or clear memory in the same OSs) I have to keep my ATM around for now.
I am new to smart phones, and with the AM I am like a kid in a candy factory - and there is now way I have had time to check out all 100 apps I have installed - but I have been installing them in groups, like I should not be doing (I know, I know) and yesterday I ran into an issue (that I now have a theory about) that cause my phone to temporarily lock up multiple times. Without ATM, I'd be stuck waiting....and waiting...and waiting.... (which is what I did the first time - it took my phone almost 5 minutes to become sluggishly responsive).
The next time it happened, I managed to get to ATM which was already running and inside of 30 seconds had a responding phone.
I guarantee that you can reach a limit to the number of apps / data that it can access at any given moment, so I keep it around.
However, I am not one of those people who constantly tries to free up my memory in Windows either. Just as an example, I have 102 processes running on my desktop home computer - with lots of stuff starting up automatically and running in the system tray.
I know it is not a necessary thing for me to habitually do - but it still helps to have it there when you need it most....and that is to kill active processes that are sucking the life out of your phone.
Then again, I am not the average run of the mill computer user - so, hopefully, soon I won't be the average DROID user either.