In that case the attacker could potentially compromise your security with an analysis breakthrough on the symmetric cipher, and a passive (e.g. bend the fiber a bit and see the light that comes out the side) attack.
The QCD requires an active attack, you have to actually insert active equipment into the line, not just peel off some stray photons.
So assume you're already using multiple layers of independently keyed state of the art cryptography, and still have money left over in your link security budget...
How much is it worth to you to make it so that attackers have to insert equipment vs just siphon off a bit of extra light with a tiny bend in the fiber (assuming they have space alien grade mathematical breakthroughs)?
It's probably worth more than nothing.
Now, I would agree that any who is promoting this stuff with claims greater than "probably worth slightly more than nothing" is misleading people.
And as an area of pure research it gives us a lot of room to learn about practical realizations of quantum-sensitive systems, optics, photonics, electronics, etc. There is plenty of research with no known practical application at all. Having a really minor application isn't a great sin.
QKD only works for point-to-point links.
If PQ happens and we have no PQ classical asymmetric algorithms, QKD cannot save us. We do not have time to rebuild the Internet around hop-by-hop security in a world of QKD, and anyways, QKD can be trivially MITMed, so this just doesn't work.
Fortunately we do have some PQ classical algs. Fortunately we're not in a PQ world yet. If we end up being unfortunate (PQ happens before we deploy PQ classical algs) then QKD cannot help us.
> I don't think it's that generally interesting, but if you already have dedicated private fiber between your super secret secure facilities, -- why not?
Because I might as well use classical symmetric keying and not worry about PQ.