You do not have to intentionally leave a summoner alive to be effected by summons giving experience.
Fixing summons giving XP does not require redesigning and re-balancing the game on a fundamental level.
Eliminating XP from summons does not otherwise impact gameplay. Depending on how you wish to try to prevent 'save scumming', it can negatively impact gameplay (reloading an earlier save for various reasons, and then going through an area getting the exact same loot as before doesn't add anything to the replayability).
