I quite like the FE7 ranking style. Yes, it encourages you to do counterproductive things. That's the point! The "Play Through The Game Normally, No Obligations" challenge is not, in fact, a challenge. And while a low turn count run has to figure out how to clear chapters quickly at the expense of all else, a ranking run has to determine how to clear chapters quickly without expending all else. How to get the treasure, kill the baddies, and save the day, all without leaning too hard on overlevelled units and while keeping an eye out for opportunities to give weaker troops their fifteen minutes of fame.
As for arbitrary, well. If you and I both beat a playthrough and our results are similar by most metrics, but I still have twenty thousand gold left over and you're flat broke, then couldn't we say that I have shown superior tactical skill by using less to achieve just as much?
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I don't like this duchy. Now, it's an adventurer.