David Gerrold entertained the idea of away teams as early as 1973, in his book The World of Star Trek. He proposed the first officer lead the team, with the captain staying on the bridge.
That goes back to Kirk leading the majority of the landing parties. You have to think about "what is Kirk's real job?" Is he basically just a bus driver" taking exploration teams to their destinations? Or is Kirk the over all mission leader, not just the Captain of the transport ship?
It really makes no sense to leave the most experienced personnel aboard the ship. It took years to train people like Kirk, Spock and others, now don't use them.
Maybe the next time the ship goes into battle, Kirk should excuse himself and leave the bridge.
If something happen to Kirk (killed), Sulu was perfectly capable of returning the ship to a starbase to acquire another Captain.
So a landing party is composed of the mission leader, the best general scientist, the best doctor, and supplemented with additional specialist, add a few armed body guards.
The Umbrella Corporation wrote:
If you look on the Starship team as being explorers and First Contact guys then would you send your second string team down to the planet ...
There would be a time and place to do just this, allow junior officers to gain experience, gain confidence, missions of low level importance.
But most of the time policy should be to send in "the first team."