I think most collectors (and Im beginning to see myself as a collector, though certainly not as accomplished as you are Gregory!) recognise that although they have their collection, however far it sprawls, they will always have their core decks which they work more with.
I have about 70 "strictly tarot" decks now, maybe a few more, but I have my regular Top Ten. It changes slightly (for example, the Soprafino has just been nudged out of the top ten by the Royal Fez Moroccan). But Im quite happy accumulating beautiful decks. There´s a developing coherancy; I can feel it a mile off when a deck doesn´t quite fit into the "atmosphere" of my collection.
But I wouldn´t buy a deck because it´s rare. I just buy what I love. But then sometimes they can be rare...