Re: Can apple buy Foxconn or some other manufacturer?
The positive with building your own stuff is that a vertically integrated supply chain reduces the risk of sudden price hikes or supply shortages. That's why Samsung do it.
The negative is that it ties you to a particular production strategy (as others have said, the upfront costs of technology mass-production facilities are phenomenal, so you can't change your mind halfway) and so if you get it wrong it weighs on the reliability and cost-efficiency of your end product. That's why Apple (and most producers of most electronics) don't.
The question of whether outsourcing your low-margin production and focussing on high margin design and distribution is a good strategy (because it supports a higher margin and more flexible business) or a bad one (because it makes your business more vulnerable to the long term plans of the people who make stuff for you) is in some ways a question about the entire western strategy of outsourcing low value jobs.
That will make Apple's long-term success or not against HTC (used to make mobile stuff for other people but now mostly a consumer brand) and Samsung (ditto but bigger) particularly interesting.