the important thing to remember in any glass polishing situation is:
to polish or remove the scratch actually removes some of the surface of the glass to accomplish the task, to go too far would of course distort the piece of glass optically speaking. in other words polish to much and you can create a wave or distortion when looking through the glass.
one more consideration is the amount of time required to polish the glass. to accomplish the task correctly can take quite a bit of time, so there is a time vs. cost to replace
analysis. of course if the glass is very difficult to replace, polishing can be an attractive way to go. jmo
one other caveat... don't get the glass too hot polishing, it could crack, use plenty of water in the slurry compound and make sure the polishing pad is free of any contamination.