I think many of us suffer from the "It'll come in handy one day" syndrome". Hamster genes, I suspect. But I often get to try out and evaluate rifles for other people, and if you do come across an oddball rifle to be tested, then it is no use having to wait for months for an exotic die set to be delivered. Apart from which, the new sets can cost more than the rifle -. have you seen what, for instance, an RCBS die set costs for an Egyptian Remington?
In many cases (or should that be for many obsolete cases?) I use die mixes to reform brass for which no die set is available in a reasonable time at a bearable price. Like using 40-65 and 38-56 to make a first approximation for 9.5x47R. Good enough for fire-forming, after which all my cases are only neck-sized anyway.
So I too tend to buy unusual sets if they are cheap - after all, they may come in handy one day!