the problem with companies is that you put your feelers out, find some nice guys at $company (or maybe $company is entirely nice), and happily establish cooperation. life is bliss, to mutual benefit.
then $nice_guy in $company is laid off or reprimanded by $bad_guy, or $company gets bought out by $idiot_company. lots of time is wasted, lots of promises are broken, lots of porcelain is smashed.
copyright assignment can work well with $company. but a whole body of IP under one copyright will give $idiot_company funny ideas.
mark's position is quite insightful, but i don't see him refute this issue, which is at the core of things as i see it.