Sometimes you need to make a second hit of your AoE before moving to the next pack because it missed something due to the enemies being spread out and sometimes you have a BLM who hit Foul on the pack while you're still rounding it up. So sometimes you need to get that second hit in unless you want the DPS/healer to hold aggro while you're running.

Leaving before the pack is dead is just annoying. It's a bad chain pulling habit that's honestly not as useful as people think. The last enemy moving costs the group DPS and it's generally nice for the group if the healer can get a tick or two out of combat between pulls.