I've made about three private treasuries. I make them so that I can keep track of things I actually do intend to buy - I tend to just add things I like to my favourites, and if I add things I want to buy to that and to that -only- might lose the links among other items, and if I keep internet tabs open I may lose them among the rest, so I use treasuries for that. Though, obviously if I intend to buy something, I'll favourite it too, it's just good manners :P
I've also, lately, been using private treasuries to keep track of different gifts I want to get some of my friends, so that I can very quickly and easily compare both the products and the prices on one page. If I made these treasuries public they wouldn't look very artistic :P
Basically, private treasuries are good for list-making ^^ I only make public treasuries for art's sake.