Very true, but only for those who get the most enjoyment from collecting that way. Others, like me, prefer F-AU coins. I know a collector who specializes in holed coins (post-mint damage holes, not integral design holes.) The cardinal rule of true collecting - be it coins, sports cards, antiques, cloissone, chess sets, or prune pits - is collect what you enjoy collecting. That's different from the cardinal rule of investing in collectibles - collect what you believe will quickly increase in value. There's just no "right" or "wrong" way to collect coins, or anything else.
Of course that requires you to predict what a lot of other people will start to enjoy collecting. You need an increase in demand for a corresponding increase in value. Why some people feel they can predict some new hype or fad when "investing" in coins/collectibles baffles me. If one sticks to what they enjoy, they will never have buyer's remorse if the value doesn't go up (or worse, drops).
Specialization will help to further the joy of a collector. Whenever a new piece that fits into one's specialized collection, there is a sense of achievement and joy. Specialization will also help to further understand in-depth knowledge of your collection. You may even found some unknown varieties of coins or banknotes or information which have never been documented. This is very true for past coins and banknotes or even ancient coins and artifacts. I will compare this to great discoveries such as going into the deep forest of Borneo or the Amazon, you spotted an unknown animal such as the recent unknown species of leopard. If you specializes and work as a Marine Biologist, you may explore the deep ocean and discover a whole world of unknown creatures. It is just a comparsion although we would have discovered most of things in coins and banknotes by now. The benefits of specialization will be still outweigh one who do not specialize.