I think the problem is that people against task killers only point out the worst case scenarios, which in normal use are not that common, so most people won't run into issues using task killers.
THAT SAID: if using a task killer does improve battery life, that is an indication that you have an app(s) that is hogging the radio and/or CPU, and it ought to be identified. Auto-killing helps stop these rogue apps, but fixing/removing the app is the much more efficient solution.
You'll find that if you just fix the root cause, task killers become completely unnecessary. Unfortunately, sometimes it can be very hard to find the culprit. I use System Panel, which shows me every app/process running, the CPU usage for each in real time, as well as sorting apps by CPU usage over time (history).
If you're still not sure if you have an app(s) that is draining your battery excessively, power your screen off for an hour. Then turn it back on and see how much the battery has drained. Don't do this test from 100% charge, since there's usually a quick falloff due to the battery indicator's inaccuracy when topped off. If you are running stock, you should see no more than 3% drain. If you are rooted with crapware removed and a ROM that supports CPU throttling or HAVS, you should see less than 1% drain per hour.
Note that if you have weak cell signal, your battery usage will be much higher, and in this situation, task killing won't help you either.