The value of the slab is not monetary per se, rather it is related to liquidity. If you have a raw MS65 Morgan Dollar that you want to sell, in order to realize MS65 money, you first need to find a collector who has the grading skills and confidence to recognize that your coin is indeed an MS65 and pay you your asking price. If the coin is graded MS65 by NGC or PCGS, you only need to find a collector who is looking for an MS65 Morgan Dollar as the grade is not in question. Since having a coin professionally graded makes your coins infinitely easier to sell, the "slab" certainly qualifies as value added. For those who think slabs are silly or unnecessary, I have a question for you. How many coins have you sold?
The labels shouldn't but do matter. Look at the premium paid for the first sold $100k gold Kennedy. Even others with the ANA show label were generating profits at one time. I don't know how the market is doing on those now, but I'm guessing many are buried.
If you have, say, a Pete Rose autographed edition PCGS slab of a Baseball Hall of Fame high relief silver dollar graded as PR-70 DCAM, and Pete Rose passes away, then I can see that going up in value, at least in the short run.
Believe the debate here is about the cute little labels put on slabs and not about the slabs themselves.