1. If we allow password based logins, there will be many orders of magnitude more login attempts to root than any other user. So if you have to allow password based logins, you pretty much never want to allow root login.

2. If we disallow password based logins, a user account would be as save as a root login, except again that the root account is the much more valuable target so will get much more attention. I also do see the relevance of cronjobs (root does run them as well) and naturally no user that has sudo privileges should be be running network exposed services.

3. In cases were admin rights have to be shared amongst multiple users, are you going to share the same key for all users (probably not a good idea) or give every user a separate key (making key management a bit of a nightmare, user management is much easier).

4. As you pointed out yourself sudo gives you much more fine-grained control over commands that can be run.