Jokes aside, soft raybrush is actually quite important, and it covers case where even live dragging of render region won't help. Because if you use soft raybrush to get rid of some last area on the rendering that just won't converge no matter what, like for example those good old highlights in strong DoF defocus, everything will be fine.
If you use the feature on that video - live render region, then you will probably be screwed, because it will leave very sharp transition between super clean contents of rendered region, and still slightly noisy surroundings. So assuming you did not use denoiser, then you will either have to retouch that area somehow in photoshop, to cover that sharp transition, or you will have to keep wiggling the region constantly slightly around the edges to create that transition manually, but i am not sure that would even work.
It may seem funny at first, but it's not that funny if you are 6 hours in rendering some print sized image, and you realize you just ruined it by using render region. Of course you can still close the region and wait for the rest of the image to converge to same noise level as the contents of that region... if you have another 6 hours to spare ;)