Well, at least in my day job, a very common question is "how much more data do we need to collect?" or "how long will the experiment take?". A response of "The analysis will be Bayesian, so those questions don't apply" is not helpful. Planning matters!

Even a Bayesian with a proper prior can make good guesses about sample sizes, for example, by saying that they have a goal to reduce the length of the 95% credible interval for the most relevant parameter by 80%.