The problem with your argument, John Hardly, is that in certain situations the 'what it may lead to' argument is not only valid but compelling. In general, it always fails. But for a wide range of specifics, there are well funded people just waiting to exploit any hole in the dike.
And consider the middle ages. At the Sorbonne, the viva voce examinations for doctorates of theology were life threatening experiences. If the questioners could show that your ideas led to heresy, it was either lose your degrees and poswsible go to prison or die.