Yes, that is the question here, and it's a really fascinating one. How often do you send two identical 128-bit chunks in practice? How much does it depend on the quality of your webcam and your lighting? If the video is grainy and splotchy in the background, you're getting a lot of randomness in your data. How much does that help? Any at all? Or are you still totally screwed?

> even if it is, why chance it when you can use CTR mode and a random nonce, with practically zero overhead?

1000 times this. Crypto code should be like the safety components in a car or an airplane. You wouldn't buy a car with seatbelts or brakes that work 90% or even 95% of the time. In the same way, you shouldn't write network code that probably doesn't let an adversary figure out everything you're sending.