In a two party system like we have with very far opposed parties, going to a system where the rival party gets its person in the top spot if the president dies unwise, as it creates a very strong incentive to political assassination and means that the country can have dramatic policy swings without election in the event a president dies or has to leave office for any reason. Plus, the president and VP do have to work together to some extent and that is much more difficult if they are completely opposed to eachother.
NotTheOnlyGamer