The important point about Tone Curve IMO, is that it operates relatively onto the live current results from the Basic panel.
Thus depending on what has happened or may in future happen in Basic, the absolute 'meaning' for any midpoint for the Tone Curve will effectively shift around AFAICT. Both in terms of the underlying photo, and in terms of anything produced as output. All we can really say with certainty I think, is that a zeroed Tone Curve plot (straight diagonal line) applies no further change onto whatever otherwise happen,s in the same way that the parametric version of Tone Curve applies no change when all tonal zones are zeroed..
But what is to constitute "the overall exposure"? If an image shows a low-key tonality then a "pivot point" should probably sit somewhere below the midpoint, in order to leave a subjective impression that the overall exposure has not been changed when contrastiness is increased. And: above the midpoint correspondingly, for a high-key tonality.
As I see it, It's a purely visual judgement whether an overall photo has darkened or lightened, or not, because some tonality change is made (with whatever intention). I'd say the Basic panel Contrast adjustment is more predictable in this sense, than Tone Curve can be.
Furthermore: the "RGB space" in which Tone Curve operates will mean more disturbances and changes in perceived dominance of hue, whenever contrastiness is manipulated via TC - compared with when tonal contrastiness is manipulated with the Basic panel controls that largely separate out matters of tone, from matters of hue..
That's been my own experience, iat least.
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