He's a fighter pilot. He has no knowledge of foreign policy except to shoot down the enemy. How could he advise on political matters of the 23rd century when there are hundreds of planets he never knew existed?
Chauncey Gardiner from Being There would be a better advisor.
Kirk is just a starship captain yet he has put his nose into political affairs more than once...even to the point of rearranging whole societies because they didn't fit his beliefs.
Spock and the rest are required to formulate opinions on planets and species they never knew about before either on a weekly basis!