The primary purpose of most of the
TPG's today are to narrow the grade down for those who have no clue how to grade. Keep in mind that the majority of expensive coins (you have to use your own definition for expensive) probably are, or will be slabbed.
The very high end buyers who don't want to depend on their own idea for grade and don't want to learn how to grade still have a decided influence on market prices.
That's to say nothing of the chance of making poor decisions, even if you know how to grade, in regards to fakes, counterfeits, cleaned, tooled, or retoned coins.
So there is a place for
TPG's that isn't likely to ever go away. As a rule of thumb, I usually don't buy coins over $200 unless they are from NGC or PCGS. Granted, they are not always right - but neither is anyone else. And from the decades of buying and selling coins, it is so much easier to do both if they are slabbed by those two companies.
As far as CAC is concerned, it really isn't much different than the star and plus that is given by PCGS and NGC. Too, if you look at enough slabbed coins in the same grade on the same date/mm it is likely that MOST folks WILL find that the ones tagged as CAC DO have better eye appeal. Not necessarily an itty bitty bit better grade, or the top end of a grade, or almost to the next grade - but simply more appealing.
Last year at our local coin club I set up a display with 40 NGC and PCGS coins of varied types and from a great range in dates. I covered all but the coin itself, obverse and reverse, and asked for opinions as to grade. We had 41 people try their grading (it was our Christmas party, which explains the larger attendance). Only three of the 40 coins had CAC stickers, but all three received an average of 1.8 (nearly two full grades, all MS) above the actual assigned grade.
That is particularly interesting since all but six of the other 37 averaged slightly less (about .3 of a grade) than the slabbed grade.
I'm fully aware that this was a small test with a small sampling. Still, it did help for us to realize that
TPG's in particular were not so far off the majority of the time from our own opinions. And, in this small case, the CAC's did markedly better.
Note I do NOT specifically collect CAC myself, and I do not let it influence my own buy price (the coin itself does that). In the above example less than 10% of the coins had CAC and that's also true for the rest of my inventory and collection.
BUT if all I have to go by is a pictures on the bay, CAC helps me know I'm not likely to get something ugly.
Posted 01/08/2016 10:52 am
THE NEXT BIG THING!
Now that I think about it, I am a CPA...this could be a lucrative undertaking!
With her approval of a coin, and her number of fans, the sky's the limit. 
