Could the advantage be that a military monitoring installation of cameras is less detectable than a high-frequency radar monitoring installation? My understanding is that active radars are easily detectable, but perhaps cameras could be less so and that advantage could allow them to be deployed closer and might require less protection themselves.
Yup, that's certainly one. I see this technique as simply one more tool in the toolbox. Air defense (and really, all of warfare) is not a single "this thing is better than the rest" but a system of things that work in conjunction with one another. This system has advantages (passive, cheap, portable) and disadvantages (short range, clear weather only). It's good and useful work, and could conceivably be part of a larger system.