What you're saying is how interviews should work: a pure talent evaluation, unaffected by any non-job-related skills. However, in practice, interviews don't necessarily work that way. Spending a couple of minutes training on presentation skills can make you much more effective at communicating just how awesome your actual abilities are, far more than the equivalent time expenditure in getting marginally more awesome.

You can pretend that the world is perfect, or you can take steps to deal with the fact that it is not. Now tell me, which of those behaviors would you want from the programmer you want to hire? ;)