I think the reason the S&W forum says they are "worthless" is so they can get a leg up on folks looking to sell or trade.
The -#s give a person a very good idea of production dates, if it is a limited edition (for example the 25-7 in .45 Colt, matte finish, unfluted cylinder, pre-lock, "Model of 1989" on the barrel), etc.
Smith also screws up once in awhile, like when they turned out a couple of dozen .45 revolvers on M29 frames. Get one of those with a authentication letter from Smith and collectors go googy.