What prompts you to collect sets by date/mm of the same design??
I will confess to having once been a set builder, the type of collector who is all too common and collects coins of the same design with the only difference in them being the date or the addition of the mint mark. I think it started when I first began to collect and there were folders available to keep a collection in, mainly those old blue Whitman's. In hindsight it was as though I was told how a collection was formed and it took me over thirty years to see through that fallacy. Today I collect what I find attractive and historic. That's not to say those coins I once cherished don't possess those qualities, the point is that I don't feel I need 1oo examples of something to enjoy the design and history.
My collection transformation started around 20-25 years ago and has recently come to completion. Most of my standard US coinage has been sold but there's still more I need to part with. In retrospect, I think I was hoarding, a character flaw that can be very debilitating when it gets out of control. What remains are some albums, some slabbed coins and a box or two of raw coins.
I understand that many collectors, members here included, suffer the same as I did and have large assemblages of the same coin design. Understanding that you probably like those designs and want to have a complete "set" of the coins as struck, what prompts you to keep buying more and more of the same thing?? I have known collectors who have had 10-20 complete albums of Lincoln Cents from 1909-date, one guy who was trying to assemble a roll set of Morgan dollars, etc. It puzzles me today why we embark on such endeavors.