Most would argue that task killers and whatnot are a bad idea - but I can guarantee that if I didn't ahve ATM installed on my Ultimate 3.7 install, I'd be up the creek. Sans paddle.
However, automatically killing apps it probably pointless - if you need to automatically kill them, then they are probably going to start right back up - so you auto kill - so they start back ....
The pattern can be a bad one, b/c if the app is not used, it literally sinks to the bottom of the river, so to speak, and just sits there idling. However, with an auto killer, it needs extra power to monitor for apps, and then kills them - and then your app(s) start back up, which uses a lot more power and CPU cycles than an app lying in waiting at idle (almost 99% of the time this is true).
Ultimate 3.7 has one flaw so far - ATM ran and automatically killed apps without me knowing it was set to do so - I promptly closed it down after removing all auto killing features.
That being said, however, I did have to go back and run ATM to kill some apps as I was installing and configuring everything this morning - and every thing was staying open - Touitier, AIM, FB, MySpace, Amazon, eBay, PayPal....all of a sudden U3.7 started to get really slow - so I killed a bunch of apps that I had previously ran but did not need open anymore.
But, IMNSHO, I won't let a computer / CPU / phone decide for me what gets killed and what does not. If it needs doing, I do it myself. Manually.