Successful startup founders are not one-dimensional business machines, they're usually smart people who have some kind of different view on the world (which enables them to succeed in the first place). As such, you can assume that they have interesting things to say.
I don't see why you'd deny them the right to speak about topics outside the narrow band of their startup.
Sorry, I know my comment is negative in nature. And I'm not saying someone can't or shouldn't talk publicly outside their narrow band. Just pointing out that when the few public interviews a founder gets are outside their actual venture, you begin to worry about the venture.
I'm not sure whether the relationship between startup success and public speaking engagements is established. Why do you think the relationship/correlation should be negative?
I don't see why you'd deny them the right to speak about topics outside the narrow band of their startup.