Why would you refinish? Why would you worry about the finish? A Glock is not a pretty gun, it is a worker gun. Holster wear shows just that.. this gun has been used.. I would be more apt to buy a used Glock with holster wear than a one that looks fresh...
Sort of a, "why are they getting rid of this?" Rather than, "sucks they had to get rid of this."
With other guns such as 1911's and Smith and Wesson revolvers as well as select rifles and shotguns, condition is everything, BUT it is always dependent on the gun
With a gun such as a Glock, XD, M&P, etc... the more holster wear the better IMO, as long as the gun is free of rust, pitting, and is in good working order.