Just save the RP. Eventually, you'll use it to buy something that is discounted or limited.
(I can't imagine anybody ever buying a key when they could just wait... a day? I seem to get one or two fragments every time that I log in... I have way more than I can possibly use. It's true, when leveling a new account, I have a key deficit for a while, but before a year passes the situation is permanently reversed...)
So, in the end, much depends on your own priorities and preferences. Personally, gambling with chests doesn't make a lot of sense to me; the available creative content that I consider desirable is limited. But others have broader needs, surely...