Actually, I thought a lot about it, and you hit the main purpose of that idea: remove the need to grind. For girls at least.
When there are no more girls, the boss will drop an affection boosting item, which will raise it by the same value of the boss level. No more time, and/or money, wasted for just a small chance of obtaining a girl, instead every battle will be more helpfull than giving just money. And there will be two ways to increase the affection faster.