Earlier quoted context omitted.
In the context of evaluating a tax, the ability to avoid it and the difficulty to apply it is definitely one key design concept. A tax on people that eat their brocoli would be highly impractical!
You're correct in that it's foolish to institute an uncollectable tax, but I don't think inheritance tax necessarily falls into that category any more than, say, capital gains taxes.
There are some definite difficulties in implementation. But even then, my argument is that it could be a really harmful tax if it could be applied at all.