Posted By:
davidcyclebackI think the essential grading problem is that postcards aren't trading cards. In many ways they're similar to trading cards, but they aren't trading cards. To start, a 1933 Goudey wasn't made to have a letter written on it and to be dropped in the mail box. So grading postcards exactly as if they are trading cards doesn't work exactly.
Whether a postcard is better unusued or with letter/postmark is up to the individual collector. A baseball card collector may collect them as cards, wishing them to be unmarked mint condition. Others collect postcards as historical artifacts, and a relevant letter about WWI or the U of Michigan football game adds quality. A collector may prefer a unique old postcard be postmarked and stamped so they can be confident of the age. Those are all valid bases for judging a postcard, and a PSA or SGC grade shouldn't be considered the arbiter for the question.
From a practical standpoint, one reason the hobby knows the Pinkerton postcards are from the 1910s, and not much later as some worried, is that a few of the blank backs were used as postcards and have the 1910s postmark. The postmarks/letters proved they were from the 1910s.