And Apple is one of the most successful product companies of the last decade.
One big thing they would get from Apple is international experience/operations experience. From Apple's perspective I read today that Tesla is working on a facility to double the worlds lithium ion battery production - that would benefit Apple.
>> "Furthermore, Tesla's entire software ecosystem is already mature, so there'd be no gain from Apple on that front."
Integration. Integrating peoples smartphones, tablets, computers, cars, TV's. Tesla doesn't have the best designed software either - something critically important if you're accessing it while driving. I'm sure Apple's expertise would be useful there.