So in that sense it wouldn't hurt to have a powerful app killer in your Android mobile phone. Although there are many but one it particular is very popular among the users it's called Advanced Task Killer. The app killer has main two versions, one is absolutely free and the other is not. But the only difference between the two is that the free version display ads and the pro version does not.
So if you can handle few ads (not that annoying) then Advanced Task Killer is an award winning task ender for your Android device which lets you do things like...
*. Runs in the background and just tap the widget and it's on! :).
*. Change height.
*. You can either kill apps manually or let it do automatically but the developer suggests that you do it manually for better stability which makes sense since computers and applications are still pretty dumb after all :).
*. Once launched it shows all the running applications and lets you kill all the apps that is checked in the list by a single push of a button (but make sure to leave the system core-apps and services).
*. It has another list called "ignore list". As the name suggests you can move apps to this list that will be "skipped" (to get this you'll have to do the long press).
*. Requires Android 1.5 or higher.
You can read get the application from this market page. But unless you're an experienced user using app killers can sometimes do more harm than good. For instance, say that you killed like 10 apps and didn't know whether they were necessary (ones attached to the already running major apps, etc) then the OS has nothing but to restart the apps thus taking away your CPU/RAM or system resources in general thus it'll even reduce battery life and unstable environment (crashing "related apps" and services, etc).
This however not always true since if you know your way around Android :) and knows what you can get rid of safely it can actually help to reduce some of the OS load which leads to a stable environment and enhanced batter life.
But as said, if you're a beginner and don't know much about what and what's not needed, etc then it's better to leave the OS alone. It's your choice mate! :).