The point of DRM is precisely to make sure that it does take time and effort to capture and redistribute video. So live events are actually a perfect example of where DRM is useful.
And we're both assuming that the people running those mirrors won't work out a way to disable the watermarks in real time. I've not read up on digital watermarking technology, but it's hard to imagine that it's bulletproof.
And yes, I'm assuming that robust-enough watermarking methodology is possible. I don't think it has to be bulletproof, I just think it has to be good enough to enable cancelling enough stream consumer accounts to deter casual mirror-ers. DRM isn't bulletproof either; watermarking is preferable for users because it doesn't require that the stream producer take control over your machine.