So I like the idea but it is not that good game sense wise. I like damaging Nexus more than last hit cuz otherwise everyone would go full attack damage even if it is only 5 bonus ip. The problem to all of this is this:
Lets say your team and other team have fight, both teams lose three champs. Your team has the ad carry and mid laner while the other team has ad carry and support or whatever. With just the enemy nexus and 1 tower remaining you guys know you wont win unless you kill the rest of the enemy team. So your mid tries to kill the two enemies getting a double kill but dieing in the process. But meanwhile your adc farms minions or goes for tower. After the mid dies and enemy team is aced your adc gets the win. Even though it was your mid laner that did most of the work only your adc gets the bonus gold.