Yeah. I disagree. I feel players should be able to build a staircase up to reach the ceiling so they can interact with it if they want to. After that, they should be able to remove the ceiling (rather than continue the staircase through it) if they want to, opening up a 'pit' above them. Doing this should not make the access stair that they built to the ceiling magically extend itself the rest of the way up through the gap so every wild animal wandering around above can wander down into their fortress. That behavior is both unintuitive and unhelpful.
Dwarves should be able to dig out channels, pits, and so on from below. It shouldn't be impossible or deliberately difficult.
Under your suggestion, it would still be possible to dig out areas from below, yes... but you would have to first place the staircase, then precisely after the channel had been dug, you would have to remove the staircase. And you would have to hope that a dwarf gets around to removing it before a wild animal wanders in, because what you're asking for, basically, is "take away the ability to build half-complete staircases for use as scaffolds."
That's not a good way for it to work. The benefits to your suggestion were, IMHO, never very good -- all you're doing is obscuring the way floors/ceilings work by having them automatically removed, which is a bad idea IMHO -- players will have to deal with them eventually to do any complicated digging. And the idea of constructions that extend their effects into two levels is something that I find inherently bad. I don't like the way ramps work, and I don't think stairs should work that way.
But all that aside, what you're asking for is something that would introduce another step to digging pits and moats from below that, in effect, would cripple it the way channeling out areas from above is currently crippled -- forcing players to designate multiple steps at different points. That's not a good idea. You're asking for the interface to be crippled in a small but significant way, one that would seriously impact the way I play the game.
You asked me to come up with something that your suggestion would break. I explained it to you. What else is there to say?