Andy wrote:
...I know of nothing a ground instructor can do that a flight
instructor with equivalent rating cannnot.

Nothing major, that's for sure. All I can come up with that CFI must
log/record each student endorsed for taking the knowledge test (as well
as the score received) and that log must be kept for at least 3 years
(FAR 61.189). A ground instructor is under no such obligation. My
understanding is that CFI endorsed knowledge test failure counts against
the CFI just as a practical test failure would. I expect that this may
be an issue when you teach large groups and can't track an individual
student well as you could if you were also his/her CFI.

IMHO, I recommend that all prospective CFIs also take the Advanced
Ground Instructor knowledge test around the time that they take the CFI
knowledge test. It's the same test except they can ask you questions
about all aircraft types on the AGI test. Really no big deal.

Tony V. CFI-G, AGI