It seemed to me that this wasn't a command as much as a recommendation. Mab's death would definitely break any contract just as Harry's death would do likewise. Molly is a sensitive who hasn't responded well to such things in the past. Granted she has grow loads since then but how well would she endure feeling thousands of deaths through her banner coming from the gates? Imagine what millions of deaths over a thousand years has done to Mab then imagine Molly feeling that. Over time she could build up to that but right now, maybe not. I think Mab in her conversation with Titania has now made arrangements for Summer to take over the outer gates should Mab fall but that didn't happen until after the battle.