After five pages of comments I don't know that anything I have to say on this subject will be of any particular value, but I will go ahead and say it anyway.
Sooner or later, unless destroyed or donated to a museum, all art objects eventually make their way onto or into the secondary market. Even the most avid and knowledgeable collector does not live forever.
All things being equal, when that event occurs, the object with a verifiable signature/manufacturer's mark will in almost every case, demand a premium. Look what happens at the great auction houses when two comparable works come up, one signed, with impeccable provenance, the other unsigned, but "in the manner of." Spectographic analysis, X rays, radio carbon dating, brush-stroke analysis, along with every other test known to mankind, can only disprove or cast doubt. Is it from the hand of the master, or his student? The potential buyer can never know with certainty, and the price he or she is willing to pay will reflect that nagging doubt.
In the field of glass, it is even worse. I am certain that all of us including the experts on this board, have inadvertently mis-attributed items, not once or twice, but numerous times. Small wonder the casual collector does not feel comfortable without a permanent imprint that announces to the entire the world "this is a genuine ..."
Can we demand that all artists and manufacturers sign their work? No.
In years to come however, I firmly believe the unsigned product of such men's labor will never develop the type of following and collector interest it might have otherwise. They deserve better.