Curious why they left it out, since it seems to be very little extra work. Same game mechanics as a single-player, non-simultaneous turns. Just need a mechanism to say that as it rotates through the turns, bring the player interface up when its a human players turn. As you cycle through the players, you litterally just check to see if the player is AI or Human, and then kick it to the AI routines if its AI, or let the player do their turn if they are human.
Its very simple programming. I'm playing with a game program of my own, and its one of the very first things I put in. Mainly so I could see the game from all sides as I develop it. As soon as I started to work on ideas like Fog-of-War, I put in this so I could see the map from different player's points of view. Its very simple programming.
Very strange that this was left out. To me it almost seems mandatory for testing and debugging. Oh, wait a minute ... no one did any testing or any debugging on this program.
Of course, this is a very strange game and was obviously a very strange development process for a Civ game.
Leaving hotseat out would lower my grade for Civ5, except you can't get lower than the F the game already gets from me.