The statistics on grading can often be skewed because of pricing on grading.  Below Mint State for many coin series it is not worth it to grade the coin because the cost of grading will exceed the value of the coin.  Only with studies like this one, or perhaps for Lowball or Everyman collections would a collector seriously consider going to the trouble.  As this chart shows this logic follows for Stone Mountain Commemoratives where there is a concentration of grading from Mint State 63 and higher, with very few coins receiving a plus grade.

For the Stone Mountain Commemorative there are no certified Good-06 coins.  From grades Good-04  through Very Good 25 there are so few that find them for sale would be unusual.  So to build a complete set for this study some additional attention to lower grades will be needed.  To build a set that included the Double Die varieties will be especially difficult.  But such challenges is what coin collecting is all about.

Commemorative coins fall under this consideration even more so than other areas of collecting because of lower demand than a series such as Morgan Dollars.  Some commemoratives were hoarded by collectors in mint state and a disproportionate number of coins exist in upper grades.    Also with commemorative coins the very top grades have very subtle differences and the average collector is much better off finding a gem MS66 and trying for an upgrade than buying an MS67, for example.

But one should not consider a lower grade coin as rare unless you get to grade Fair-02 or Poor-01.  Here the difficulty of qualifying a coin for grading can add slight value, and PCGS now recognizes that in their suggested values.