My understanding is that Android keeps your most used apps in memory to speed up opening them. There's apparently some mechanism to reopen apps even if they've been closed. My guess is that reopening them is probably more battery intensive than just keeping them in memory.
IMO, the best use of task killers is to stop an app that's gone "rogue" and is using up your CPU/draining your battery or closing an app that you want to clear the memory on (e.g. a browser with a bunch of open tabs you don't need) or some other pretty specific use. In general there's no need to close apps. The memory will be kept alive either way and the apps are suspended, not actually running. If you have apps that use too much battery it's better to either find new apps or change their settings so they refresh less often.