When the entire residue of an estate is bequeathed to either a spouse or P.B.O., you pay tax on what YOU get, not on what-you-get-and-the-tax. In other words, S.A.R.S's. 20% off is 25% on and that's a massive 25% too much tax!
Your heirs receive more, charities receive something and the only one to lose out is the taxman. Remember that in formulating your will you also want it to be robust against any changes in the threshold and the promulgated tax rate.