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hankronYour point is a good one, Adam.
For example, many of the toughest Topps odd ball issues were not finished cards (were proofs, pre-production), sold to the public and, in many peoples' minds, don't qualify as trading cards.
I think the best sets for the hobby, like the 33 Goudey and T206, have most cards readily available so every collector can join in on the fun, but have one or more rarities that make a complete set difficult.