jimkaray

Its called a wear pattern not a tread pattern actually. Were talking about hardened steel gears, you don't wear grove into them in a few thousand miles. If the wear pattern was set correctly to start and you are reusing that matching gear set there isn't a problem. You always need to set the contact location properly or gears will audibly whine, even when new. Some racing gears whine regardless due to the design.
By your logic every time a rear was rebuilt you would need to install a new gear set. If you add a t-lock and keep your stock gears do you need to replace the stock gears? Many shops won't install gears because it takes some experience and some time to do it right. Whine issues are almost always the result of poor installations. Its easier to blame some one or something else so they don't have to redo the job.

alkemist

You have to get the backlash to the exact specification as it was in the previous car to match the wear pattern. If it was my own reinstall, I could measure the backlash but not knowing that information can be critical to success or failure. And yes, gears do start to wear in once you use them. Probably difficult to detect, but the wear pattern will be there, even if it's just a light print. I'm Just putting it out there that buying a previously owned set of gears can be more of a headache to save a few bucks. Maybe you have the perfect set of gears, I don't know but some people don't know what they are getting into when gear shopping.