As for Sasieni stems, I'm not sure what you are asking. (Meaning I don't see how one collectable brand is different than any other)
Uniquely in the world of classic British pipes, values on Sasieni pipes, in no small measure, depend on whether the stem is a 1 Dot, an 8 Dot, or a 4 Dot. So if the stem is a replacement, and the artisan creates an 8 Dot, doesn't the fact of its being a replacement, rather than a factory original, matter?
And there is another wrinkle on Sasieni pipes. The successor owners released limited editions of Sasienis stamped with the old fishtail logo and the "specially selected" stamp. The stamp originally was applied to pipes that Joel Sasieni had personally selected from the production line as being of superior quality. These 1980's vintage pipes have no such provenance. These pipes show up on eBay from time to time, represented as pre-war "specially selected" pipes.
Collectibles are subject to different parameters. With regard to antique bronzes, the patina that develops over time is much prized by collectors, much like the rosy tint of a well smoked meerschaum. The age and history that such a patina represents is an integral part of the piece. Removing that patina robs the piece of its character and its history. Coins are subject to different requirements. Condition matters. But when a coin has been similarly cleaned up, it loses value just like a bronze. The alteration reduces value.
The problem with pipes happens when a seller lies about the condition of the pipe. Rather than saying that the pipes has been restored, for example, the seller claims that its current condition is its natural state. Cosmetically the pipe looks little used, but in reality it's had a busy working life. The buyer is not getting what he has been led to believe. The seller is perpetrating a fraud.
Marty Pulvers wrote about the problem with extreme restorations that make a used pipe look new, coupled with claims that the pipe is unsmoked. A 1920's era unsmoked cased Dunhill is going to be much more expensive than one that has been smoked even once.
As larger amounts of money chase rare and desirable pipes, fraud in the marketplace, both large and petty, are on the increase.