I stock reproduction parts alongside originals, and some purists don't like it. I keep them because a restorer who only sells originals can't finish most jobs, and I keep them honest by always saying which is which.
Why they finish jobs originals can't
Here's the plain problem reproductions solve. Originals are a shrinking supply, and a lot of jobs need more matching parts than the world still holds. You need eight identical cabinet pulls, or a full run of matching hinges, or the one specific fitting a fixture is missing, and the originals survive in ones and twos, not in the number the job takes. A good reproduction, made in the right material to the right pattern, lets you finish, instead of hunting for years or settling for a mismatch. Often the real choice isn't reproduction or original. It's reproduction or nothing. Safety is the other reason, and it's why I stock reproduction cord. Original cloth cord, ninety years old, isn't safe to run, so what you want is new cloth-covered cord that looks right and is wired to current standards.
The rule that keeps it honest is simple: I always tell you which is which. And I won't stock junk. There's a difference between a solid-brass reproduction cast to the right pattern and a pot-metal fake that copies the look and wears out in a year. If I sell you a reproduction, it's made of the right stuff and it'll age like the original it copies.
Salvager's Note: Buying a reproduction? Make sure it's solid brass or bronze, not pot metal or plated zinc, and make sure the seller labels it as a reproduction. Solid, honestly-labeled reproductions finish jobs originals can't; cheap fakes just fail.