Lifehacker is still recommending task killer apps? Those things are almost entirely regarded a Bad Idea™ among the Android crowd.
I can't cite a single negative side-effect of using it for the past 3 months.
Your mileage may vary from phone to phone.
The paradox is that people using task killers often force the system to keep restarting apps it wouldn't have had to restart. This is the same nonsensical behavior that has people clearing their file system cache because it's "leaking memory".
My other post has been modded down, so I'll save the niceties: Most of the people who swear by ATK do so because they are ignorant, either knowingly or because they've been misled by others.
Maybe you were modded down for being in the "knowingly troll-ish" group.
If you want T-Mobile's stuff to go away, install a ROM that doesn't have those things. Moving my stock myTouch3G to CM was the best thing I ever did; T-Mobile's image is massively out of date and sucks. Install up-to-date software, and the phone works much better.
Yeah, I know "average users" don't want to root their phone and install custom firmwares, but you aren't an average user. Use your intelligence to solve the problem instead of convincing yourself you're working around it.
And eating cheerios for breakfast makes your hair grow faster.
You are suffering from the placebo effect. You are deluded.
Now, if you install every app from the Market and don't use them, then that can be a problem, because some apps do run in the background. But that is a very special case. Things like Maps, Browser, etc., aren't eating your battery life. Things like updating Twitter every 30 seconds are. Telling Twidroid to update your tweets every 15 minutes instead of every 30 seconds will save more battery than all the task killers in the world ever will.
(Alternatively, there are some nasty apps that start at boot and won't quit, like Sipdroid. Those you have to kill with fire. But it's a special case, and not how normal users use task killers.)
The other case is the homescreen-killing issue; that is something task killers can give the illusion of fixing. But it's better to install CM, enable compcache and swap, and never deal with that BS again. I did that, and it staved off my need to spend $600 on a new phone. Task killers, in comparison, did nothing, because they don't solve any underlying hardware problems (or work around them in a clever way, like compcache).
Better yet, Settings -> About phone -> Battery use. Display and Cell radio usually dominate this list.
Oh well, sometimes a feeling of control is enough to make you feel less bad about having shitty hardware, so I guess task killers are good for that reason.