Open sourcing it wouldn't solve the discovery/hosting part, but I still think they should do it.
I'm sure it wouldn't be "easy," but it wouldn't be unprecedented.
Yahoo! folks also should take a look at dataliberation.org, for best practices on getting on data out (also a Google joint).
(Nope, I don't work for Google :D, just think they've done some good things in this space. )
A bit of data I'd like to get out of delicious that i can't right now, is social stuff/ relationship stuff. It really helped w/discovery.
Though you're totally right: link stuff is thorough.
Sadly corporate responsibility is not the norm, though it should be.