Have you ever tried the layered approach with something where the foot placement is a little less flexible? Like walking down stairs. Maybe it doesn't make any difference?
Generally, the foot placement in any scene is pretty well thought-out and planned in advance, but you're right that some scenes are more specific and others can certainly be more flexible.
However, the "hide the legs" layered approach isn't about just animating the upper body and then letting the feet fall wherever they may. You should pretty much know exactly where those feet are going to end up and exactly what your poses are going to look like. Where the layered approach helps more is with the TIMING of the feet rather than the position or posing. If the upper body feels correct in its timing, then that can help you know when to lift and plant the feet, but the actual posing of the legs should be something you've already planned out ahead of time.
Going downstairs is particularly exact, I suppose, but I think the layered "hide the legs" approach is something I would still likely use in a scene like that. I recently did a scene of a character climbing up something somewhat steep, and the foot placement was relatively inflexible, but the process seemed to work fine. It did require a little bit more back and forth than normal, though.
For something like going down a series of stairs, I would probably still hide the legs, animate the upper body going down the stairs until the timing felt correct, and then show and animate the legs. Most likely, I would have to readjust the upper body slightly here and there to accommodate the legs, so there would definitely be some back and forth, but overall I think it could still be useful, and certainly simplifies the blocking process.
And again, the "hide the legs" approach is something I would only recommend to those of you who have been animating for at least a few years already. You need the experience to be able to visualize ahead of time how the legs are going to be working, where the footfalls will likely be, etc.
Hope that helps!