I like the current system but could also see that working as well. Or you could do 10 for first king captured 15 for second and 20 for 3rd. that would make for a new plan of waiting to take the first king (guarding it) until another king was taken or not risking a bishop on up to take the first king with reality of losing it.
Either way I think the game plays out well.
When a player resigns in the opening of the game, one of the remaining lucky players can capture the resigned king and gain an easy advantage.
Perhaps it would be more fair if resigned kings offer +10 upon capture, and any player's resignation will take effect 10 moves after resigning.
What are your thoughts?