2) If you can't decide what to do with your money, you have less incentive to work hard for it.
3) If your choice is either to spend your money frivolously or have it expropriated, you'll spend it frivolously.
4) Distorting savings and investment decisions is one of the most damaging forms of taxation. A tax on inherited wealth is a tax on savings.
5) It's very difficult to write down a theoretical utilitarian welfare model that suggests an estate tax is a good idea. (It's easy to write a model where inheritances are optimally subsidized.)