I'm glad you find HathiTrust useful. I just want to understand what you mean by "some items seem to only be searchable with a logon from a partner institution." Anyone can search any item or public collection, and no one -- even HathiTrust partners -- may view in-copyright works, even with a login.

I've found even the search-only titles useful; by seeing how frequently my search terms appear, I can decide whether I want to try to ILL copies of books our library does not own.