in that case, this is still probably a good feature for you since you can make an encoder that just doesn't re-synthesize noise and you'll get a de-noised picture.
Not really, because these encoders are (generally speaking) making pretty large compromises with their noise filters in order to be timely. I want to spend the extra time doing motion compensated denoising.
These noise filter will sometimes be temporal, rarely will be motion compensating (because that's computationally expensive) and as a result can't get as good a result as I can.
