Originally Posted by Netizen:
“Could you explain why you disagree? Do you believe that all apps behave as they should 100% of the time and never cause performance issues?”
It depends on what type of app it is and what you intend it to do. Most apps/games are just activity based ones and they get suspended when they leave focus, then restored when you go back in. While suspended they're not using any battery or proc, maybe a little bit of memory to store their "icicle" but that's managed by the OS and will be reclaimed in low memory situations.
The ones you have to watch for are apps with services. Services are background processes and these are the type that evade task killers as they normally restart themselves. Basically if an app needs to do something like checking a webservice, checking battery levels for widgets etc.
Even with services it's not that simple, to prevent the phone from sleeping the service needs to acquire a "wake lock". This requires it's own previllege so you should be able to spot any potential offenders as you're installing them.
Also, services use alarms to wake up to do there background work (how they get around task killers as they are sent a message and the system will start them if they are the registered receiver of a message). By default getting a resolution of under 30 minutes is not easy and the OS will decide if and when to wake up under that. Also, if you don't have the wake lock permission you can't ask to be woken while the device is sleeping.
What really sucks battery big time is connecting to the internet, so apps that poll like some twitter apps, site pings etc are bad for battery. However, as above they evade the task managers and can't run more than once every 30 minutes. Most of these apps are moving to Google's push notifications which has no significant impact on battery life.
The real battery killers are games, anything that uses the internet, GPS etc and when they are sucking up battery it's because you're letting them to perform a task.
Android really has no need for task killers and any advantage you get is partly confirmation bias and partly you temporarly killing things that you have installed to run the background and will come back anyway. Most of the "articles" I read saying you do are out of date or have little real facts in them, which is amazing considering how easy it is check this stuff.