Madjic
I am going to be honest. I don't know how a loss would have been more useful. The enemy doesn't have trenches of their own. And there is no maze of trenches to fight for. The only loss they can have right now is secret knowledge that her side is keeping secret. Another loss could have been a flank that got beat back in a way for them to learn from. She needs all the wins that she can get. She has the knowledge to prepare to some extent the weakness of her type of defensive battles.
A loss could be important to an invading army for a variety of reasons. The main reason that she is thinking that she might have perfered a loss here is because the Morale among the Main army lead by Isabelle, the War Chief, is pushing hard to attack. This would solidify their want to attack the wall before they are ready too. Sometimes too much confidence will be the very thing that leads to a fall. That is what she is thinking in this. I may not have gotten that across well.