In my experience, a well considered disclaimer can be a very useful tool. For example, let's say that Mike already has a large estate and doesn't need the funds. If Dad's will gave property to people Mike would otherwise like to see benefit (say Mike's children), then a disclaimer probably makes perfect sense because he does not need to use any of his gift tax exemption to get the funds to his children or increase his estate tax liability by dying with the funds being part of his estate.

Of course, we are all just theorizing because we don't know more about the situation!