Not who you replied to, but I, personally, think it boils down to trust. Using someone else's code, _especially_ if it's functional, should be the same as using the proverbial black box. You shouldn't need to understand their code in order to use it with your own. The same should be true of your own code, really. You shouldn't need to remember implementation details of a function you wrote in order to use it in elsewhere in your code.

I know there's then the issue of security, freedom, openness. . .whatever. But if you're working on a project with someone else, and you don't trust them, the project's probably doomed to fail to begin with.