So why not just use Google Drive (or dropbox)?
I feel with features like secure file sharing (though only with other ppl with google accounts), reasonably good security and Inactive Account Manager it should work for legal docs. Especially considering Google is going to be around for a while.
I would rather use a Mozilla offering but they don't really have too many things for regular consumers outside of firefox and send.
Google reads (and censors, not that that would be an issue) anything added to Drive (and uses that data to target ads at you). And Docs is primarily aimed at collaboration rather than secure file sharing. And revoking permissions isn't easy. And it's all tied up in to Google identities, which may or may not be a recipient's personal Google ID rather than their professional ID - everyone has a separate work email, not everyone has a separate work Google ID.
Dropbox is designed to synchronise a folder between two devices. You can use it to share documents, but that's not what it was designed to do. And if someone deletes it off the shared folder, it gets deleted for everyone... not ideal in this use case. It also creates a dropbox folder on the user's hard drive, and will automatically upload anything in that folder, and copy that to everyone else sharing that folder... it's democratic when this use case needs to be authoritarian.
Does that make sense?