Now, we all know that geisha generally don't come into play until late game in Shogun 2. I think CA really missed out on a great opportunity here to help further immerse the players in the setting and bring a greater degree of realism to the game. Much like how the geisha in Fall of the Samurai inspire armies, these new geisha would be different than their highly specialized counterparts and would appear early in the game. You would have the option to perform for your general or Daimyo in an interactive mini-game of sorts. Think of the impact!
Certain generals would have certain likes and dislikes, so finding those out by reading their traits would be key. Going from that, you plan your performance and do your best to make them happy. Better performances end with a Daimyo or general with higher morale, and we all know that a happy general makes for a happy army, thus boosting the army's morale, as well. It's much better than simply clicking to "embed" the geisha into the stack. This leaves it up to you, the player, to use your strategic thinking skills to find out how you should use the geisha's charm.
But watch out! If you perform poorly or offend the general, it could lead to the end of the geisha's career! There is no room in Japan for retired geisha, so you must be extra careful. If a general is offended or displeased, he may take it out on his army, making poor calls in battles to come, costing morale and maybe even casualties due to clouded judgment.
This is just a base, too. The sky's the limit!
edit: Just to clarify, I am not speaking of anything sexual in nature.