@pastanaga - Well, you might get a "one size fits all" cast iron heat diffuser and solve all your heat diffusion needs in one go. You don't need a heat diffuser that is going to fit snugly on every pot, and if you get one in cast iron it will last forever.
That way you get to keep your options open when it comes to buying pots and pans in general. And you get to choose when you use them with the heat diffuser.
It can slow down the process of cooking quite a lot if you do actually want something at a rapid boil. So, buying a pan with a built in diffuser means that pan can only ever be used for a certain kind of cooking, which is a bit limiting.