Why can't we use HDR and AA at the same time?

The point is, we can actually do it with RenderTexture. But doing it with RenderTexture is tricky and easy to get bug with other image effect. (I am actually having one with the camera's viewport rect not being full screen). I know it doesn't behave well in some hardware, but at least they should let us choose.

 

Because no one at Unity has yet spent the time to get it working totally, and there are several features of Unity that end up working against MSAA already (like screen space directional shadows, or soft particles, or anything that uses the depth texture). There's also things like reverse tone mapping that should be taken into account otherwise you loose the benefit of anti-aliasing with high brightness values. I know a few Unity employees have expressed interest in getting this working, and it seems like they might be working towards this eventually.

 

They are apparently working on it now, https://twitter.com/JoachimHolmer/status/640542157548453888