You're absolutely right. Unless the ship is populated by cripples, on a "day to day basis" you'd actually the crew to walk most of the time, too. Otherwise, you'd end up with obese engineers and commanders... The main role of horizontal shafts is to provide "crossovers" between vertical shafts, and to provide "looping" to allow for reasonable traffic flow. The ship really OUGHT to have some stairwells in various places. We never saw those in TOS. We saw LADDERWAYS, but no stairwells. Well, the answer to that is that ladderways can be cut off by hatches (much as seen in TWOK) but stairwells need to be in isolated rooms to provide "sealing" between decks (in case of hull rupture, for instance). So the reason we never saw them in TOS was because they were behind closed doors! The other issue with lots of horizontal runs is that you create these areas of the deck which are inaccessible from other areas of that same deck. But there's an easy solution to that. Assume you need a complete "loop." Simply stagger that loop, so that (for instance) the forward and aft quadrants of the circle are on Deck 6, and the port and starboard quadrants are on Deck 7, with four vertical shafts (that's assuming that the 45-degree placement of vertical shafts is used, as opposed to the 90-degree placement which is more implicit in the TOS design, of course). You need very few horizontal segments. JUST ENOUGH to allow for "no roadblock" transit between the different vertical sections, and the ability to reroute and provide access to the key decks if a section of shaft is disabled. The rest of the time, the crew NEEDS to walk!