Automatically killing tasks doesn't always cause problems. Killing stuff that comes with the phone is often where people run in to trouble. As far as killing things that you've installed:
- why not find alternative apps that don't run when you don't want them to instead of having to kill them?
- if an app starts up again by itself then killing it is perpetuating an endless loop that uses battery and will slow your phone
- if an app is sitting idly in the background, there's no reason to kill it as it is using zero resources and Android is smart enough to kill it as necessary. An autokill app of your own just duplicates the effort, using resources and opening up an avenue for problems.
If you're happy with it, then that's fine. I can imagine certain circumstances (if you've installed poorly designed apps, for example) where an auto task killer is necessary. Also specifically on the Droid X after the Gingerbread update: