[rimantas]

  > It's just alarming to me how little being a better
  > speaker depends on making your ideas better.

It's not about making ideas better, it is about getting them across better. If your idea has value of X, then speaking makes it aX + c, where c is some entertainment value, so the speech can be entertaining even if idea is worth zero.

However I do think that spending some time thinking about how to present your ideas in oral form can indeed help to improve them.