An "intermediate" staged commit is special, as I have tried to explain many times now. If you stage a commit by using git-add to include only some of the changes in your working directory, you are committing a filesystem state that never existed in your working directory prior to committing it.
You can disagree that this matters, but you cannot disagree with the above statement. It is simply a fact.
Let's walk through a scenario. Suppose I am at the point in development where I actually want to create the commits that I will publicly push (ie. this is not a "commit early, commit often" scenario). All of my final changes are in my working directory, but for cleanliness I want to break them up into several commits by staging them.
$ git status
<git prints a bunch of "Changes not staged for commit">
$ git add foo.c bar.c
$ git status
<git prints foo.c and bar.c as "to be committed", baz.c
is still "not staged for commit">
$ git commit
I do have a couple of options now. I can stash my changes to baz.c and run the tests; then "stash apply" them if the tests pass. This is probably the best option for ensuring that my staged commit is not broken. I can also commit my changes to baz.c, then checkout HEAD^ and run the tests, but this solution forces me to mentally keep track of which commits I have tested; if there are many commits in the series, this is burdensome.
Both of these solutions are entirely possible; I'm not denying this. The "stash" solution is probably the best option for this. But that said, it is fundamentally true that when you (partially) stage a commit, you can't test your commit prior to committing it. So when you write your commit message and mentally perform the process of deciding that you like this commit enough for public consumption, you don't actually know if your tree is broken or not. Even though there are workarounds, I still think this is a bit clumsy.