Ultimately this is an economic argument. With about $10k you can buy a few dozen machine vision cameras [I would say "webcams" if I knew less about webcam compression], a PC with mobile modem, and a self-contained solar power supply. For $10M you can buy a thousand of these. A thousand of these evenly distributed within a few hundred kilometers of your country's borders are going to add dramatic capability to your air defense network compared to going from 5x $10M radar systems to 6x $10M radar systems. The key bit is that $10k costs far, far less than a single strike missile to take out one of these optical detection sites.
A full Patriot battery is $1100M, with the durable radar and command components being more like $400M.
Add another $10M and build passive radars from consumer stuff that listens for ruccus in telecom and television fields and you have a pretty annoying defensive baseline.
few old iphones or android smarthphones would do the trick even better - they have much better cameras than webcam, old iphones can even record at 240fps. NPU, GPU, LTE, GPS, battery, IMU, stereo microphones also already included in one package.
not really as most modern airborne weapons platforms operate bvr, even if they were on the horizon, and your array of cameras were operating in optimal conditions there is a still a hard limit on the effectiveness of such a solution.
For offensive work, yes. If you control the ground, though, and are policing a defensive perimeter against air attacks on targets in the interior of your territory, you can make and distribute the camera stations cheaply enough that it doesn't make sense destroying them.