Nice article.
Although you don't often think about it, if you buy one copy of a card with a high degree of self-synergy, you're likely to buy a lot of them.
Take Silk Road, for example. Silk Road scores more points the more Silk Roads you have. So more often than not, you either want 0 or 8 Silk Roads in your deck. Taking 1 or 2 happens sometimes, but isn't really maximizing the card.
Some other examples of cards with self-synergy include Minion, Sauna/Avanto, Fool's Gold, Urchin, Cultist, Scrying Pool, Hunting Party, Herald, Wishing Well, King's Court and Alchemist. With these cards, more is usually better, particularly for the stronger cards on the list.
On the other hand of the spectrum, there are cards that are best in moderation. Gladiator is a good example of this. It's a strong card, but most decks only want 1. And, since the Fortune doubling is limited at once per turn, you usually want exactly one.
Some other example of these support cards include Outpost, Crossroads, Militia, Counterfeit, Ghost Ship, and Spice Merchant. The first copy of these cards is often good, but subsequent copies have diminishing returns.