We've failed many rolls in combat, those rolls didn't kill us, they made our situation worse but recoverable. Failing a magic roll, especially a Battle Magic roll, puts us in Instant Danger Zone all by itself. Maybe we'll nail the recovery and zip the magic somewhere else, maybe we'll injure ourself to some unknown degree of fatality, or we roll on the Mark of Ulgu table. And we are hella running out of the good marks. Having a FC spell means we have another tool in our Standard Operating Procedure because it is safe and consistent, it's just really useful to throw a minion at someone, and we can do that consistently to decrease the pressure put on us before we are pushed into the corner. If it's Battle Magic, it instantly becomes the riskiest tool in our arsenal, and I'd question the necessity of taking that risk compared to our other options. I just generally strongly approve of Mathilde being minimal about the Battle Magic she learns, she only learns the ones she can cast safely after she gets it down. Again, only reason we learned Melkoth's was because our staff made it safe.
It does kill me to only have 1, but it's still a good tool to put in the hands of both Mathilde and the other Magic 7's that may learn it in the future. If the situation is bad enough for Battle Magics then there are much more useful ones.