There is a lot of talk in the last decade about "gradeflation", and it's effect on the coin market. Dealers and collectors are worried about changing standards, and the effect it has on their coins' value. It is this author's opinion that collectors need to go back to their roots, and stop collecting "grades" and instead start collecting "coins."
What does it mean to collect a "coin" as opposed to a "grade"? When you collect for the grade, you are buying primarily based on a coin's numeric coin grade; is it an "MS-64" or is it an "MS-65"? That is your concern when you grade-collect. If you are coin-collecting, you are collecting based on the rarity of the coin (e.g mintage of 200 coins vs mintage of 2,000,000), the beauty of the coin's surfaces (luster, toning, spots, etc are more important than the numeric grade), and also based on the history and story behind the coin.
Mint errors are collected based primarily on demand for the error type (which could be called the "history or story" of the coin), rarity and last of all, grade (also, grading standards for mint errors are much looser than they are for "regular" U.S. coins.) Error collectors are far more concerned about the coin, than they are about it's grade. They are much more "coin collectors" than they are "grade collectors."
This approach also makes a lot of sense for collectors who are collecting non-error U.S. coins. Within any given U.S. coin series, a coin's rarity should be the primary concern, and the "grade rarity", should not matter that much beyond perhaps 3 tiers of mintstate (MS-60, MS-65 and MS-70), with prices reasonably reflective of those grades, but not of that much concern. Things like surface quality, ugly toning/spotting, distracting marks, strike, etc should be more concerning to collectors than a numeric grade number.
Grading standards for mintstate coins are unstable and changing, and are actually very hard to even define, so why is everyone buying based on this unstable "grade system" of collecting? Rather than worrying about a coin grading MS-66 over an MS-65, buy a coin for it's physical rarity, overall pleasing appearance, and leave it at that. People seem to be "grade collecting", when they should be "coin collecting."