The theater onboard the TOS Enterprise (as seen in "Conscience of the King") presents some interesting questions. Basically a re-dress of the engineering set (easily recognized by the large, mesh screen in front of the cathedral pipes), the theater appears to be nearly two decks in height. Some fans will reflect this dimension in their own deck plans, including the cutaway poster done by Matt Cushman (see right side of decks 8 & 9 in photo). I'm sure one reason for the inclusion of a theater was to show the viewing audience that other areas exist on the ship besides the traditional facilities always shown such as the bridge, transporter room, sickbay, briefing room and engineering. Realistically, though, on a starship where space is limited, is it practical that such a large amount of area be devoted to a live-acting theater? It would seem that any form of entertainment on a starship would be holographic (as seen in TNG) where the crewman can be surrounded by, and actually be a part of the entertainment he is watching. This is alluded to in "The Making of Star Trek" where Stephen Whitfield describes an "exotic entertainment center" onboard the Enterprise. Actually, given the nature of the Enterprise as a science/exploration vessel with a planned obsolescence of 5 years (most of that time spent away from starbases), just how many live-acting theatrical plays would be performed in that time period to justify building a theater on the ship? It's not like the ship would be picking up actors at every planet it visited, unless crew members formed their own "talent club." If live theatrical plays were performed, it would make more sense to use a large area of the ship with enough room for stage props and audience seating. Maybe in "Conscience of the King," Kirk ordered that engineering would be used as a theater for a couple of hours?