By buffing, you are actually removing the window protective film. While the scratches presently are on the film, buffing the glass will take these out and also dramatically reduce the thickness. The next scratches will be induced on the main glass rather than the film. Cracks on the glass would mean the glass is more prone to break with temperature changes or even chipped stones.
Cheap scenario is to get a good window/glass polish, polish the window vertically and horizontally, let the polish haze. Remove the polish with a cloth. Repeat 2-3 times if its very severe. Once done, you can apply wax, sealant or even a spray on sealant. These will repeal water not allowing water smearing on the glass and even in the heaviest rainfall, you wont need your wipers.
Or else, simple replace the mirror. Remember, the roadside guys who have rotary buffing machines done have proper tools to check how deep the cut through the glass. A little over buffing and you will end up driving a car with no front screen.