On other certification types:
Apart from what David said about GnuPG, no one has ever done any sort
of certification other than the 0x10 certification signatures. (By that
I mean that there's never been a program that actually went to the
trouble of creating a UI that effectively said, "How good do you think
this certification is?")
At one point, we removed them from OpenPGP because no one used them.
They were put in because the working group liked the expressiveness of
the language even if no one was expressing themselves that way. I think
that discussion is still valid and don't see a reason to change it. The
different degrees of certification are in the OpenPGP language because
we as a group think they should be there.