61.189 specifies the record keeping requirements for flight instructors. 61.211 and following for ground instructors has no such mention for ground instructors. Of course a ground instructor is well advised to maintain records showing they meet the recency requirements of 61.217.
Others on PoA have noted before this may be the reason the King Schools endorsement for the written after taking their course is signed with their ground instructor certificate, not their CFI certificate number.
I think I have misunderstood it a bit. But it does look like you have to record everyone you endorse for a knowledge test as a CFI. But no such requirement for an AGI. Isn’t that correct?
So that might provide an advantage for King Schools or an online business, but not much for the typical CFI.
In summary, it sounds like 3 possible advantages to having the AGI:
1. Can become a Gold Seal flight instructor.
2. Only have to be current each year, not renew every 2 years.
3. Do not need to record the ID and test results of each person provided with a knowledge test endorsement.