That's right, as far as I am aware all the current plots have been claimed.
I would say it is worth getting the plot only if you plan/are able to spend the additional peds to build the buildings, if not it might be worth holding onto the deeds... Consider also that it might be cheaper to buy a pre-claimed plot than buy a load of deeds, and hold onto your non-X deeds for now (you could always obtain a second plot in the future and sell one). In theory as more and more cities are introduced those non-X deeds should become rarer and more valuable.