It makes sense to me that every stairway consists of two halves. My parents removed a stairwell in our house years ago, but left the top couple steps in place (we use them as shelves now), so I grew up perfectly comfortable with the idea of a down stair without an up stair.
I agree in game terms though, it is generally a pretty useless state of affairs.
On the other hand, I don't like games that take too much control from the player, and besides, it could be even more confusing to newbies to want to dig down, but only have an option for a stairway that goes up. Even if they had some cue that stairs are supposed to placed 1 level down, it would still render stairs almost as counter-intuitive as ramps currently are. (Stairs made
much more sense then ramps to me when I started playing)
What about a compromise?(Ignoring constructions for the moment) What if we kept all three options, but changed it so that designating each type of stair automatically designates their counterparts?
So if you want to dig downwards, you designate a down stair, and an up stair is automatically designated directly below it. Vice versa for up stairs. That way the player can just choose an option corresponding to where they want their stairway to go, and never have to know that it's made of two parts (unless they accidentally un-designate one, in which case they get an interesting learning experience).
To make things clearer, we could even rename "Upward stairway" and "Downward stairway" to "Dig stairway upwards" and "dig stairway downwards" to better reflect the action taking place rather than the pieces being used.
Of course if a more advanced player wants only half a stairway, they can always remove those extra designations manually. That means one extra step to create an unconnected stair, but it should be made up for by the fact that you will probably want connected stairs more often than unconnected ones. The rest of the time, it means one less designation, and one less chance of misalignment, so IMO it would be a good trade-off.
Up/down stairs would be slightly more complex, but not by much. They would automatically designate an up stair above, and a down stair below,
provided there is no stair already in that position.
So if you place a single up/down stair you get this:
z+1: >
z : X
z-1: <
Now you move your cursor down one floor and replace the auto-designated up stair with an up/down stair, and you get this:
+1: >
z : X
-1: X
-2: <
...and so on...
> > > >
X X X X
< X X X
< X X
< X
<
...and then you could remove the caps if you wanted to.
The same basic idea would
almost work for constructions, except that down stairs can't currently be constructed without an existing stair already in place. In other words, this last part might have to wait until the game learns how to construct things in order-of-supportedness.