Ive had about 5 different people tell me that Android is very similar to Linux and it uses resources differently than window, the apps show up as running but it's in stand by( not slowing down the OS),
If you dont' believe me spend a whole day using Droid with Task Killer installed and killing everything like I did. and then try it for a day without Task killer. You'll find out that your Droid emails get through and battery life hasn't changed one bit.
Battery life is the key with cell phones, more apps running = shorter battery life.
I tested my phone with and without killing apps and it didn't make any difference with battery life.