It's common to create a charitable remainder trust, which provides for a survivor and sends the remainder of the estate to a charity when the survivor dies. By disclaiming this bequest, the survivor immediately bequeaths the remainder to the charity, meaning the estate gets the full charitable deduction in taxes. Even though you're giving up the cash, this might be better for you (and the estate) down the line.
Even if you're not in this particular situation, remember to check for the next person in line before disclaiming. If the will provides for a charitable donation -- in the case of no living heirs, for example -- the estate will still get the tax break when you take yourself out of the running. This could help you avoid the kinds of taxes that come when you pass your applicable credit, the same way that tax-free spousal inheritance provides its own disclaimer loopholes.