CGS only guarantee the authenticity of milled coins since 1660 (which is fine because there's nothing dafter than a hammered coin in a slab), not sure about the other TPGs. People have ruined plenty of coins in order to get them to slab a grade higher. All the shiny dipped coins that for some reason the TPGs tend not to penalize... of course a couple of Viking peckmarks and it's the end of the world but a few coats of silver emulsion and no one bats an eyelid. On the other hand, try performing an analysis of the price performance of raw coins over the past 50 years and it'd be exponentially more difficult and less reliable than coins that have been slabbed. Probably fairly accurate inferences can be made about the popularity and price of silver Eagles since the 70s as zillions have been through the doors at NGC - But this speaks to a wider problem of population awareness and a consistent grading system. The TPGS only go some way to solving this problem.