Fortunately, people tend to mimic the construct of the questioner. It's still a problem, though, because even with removable hints there are some shortcuts they might be able to say. It's not realistic to give them suggestions of more than three or four possibilities, but at least they can get through the interaction. Speech is error-prone. But even when systems are not grammar-based, being able to predict what people are going to say improves performance.
Discourse is part of our old brain, of the way it has developed to understand language.
So if it seems natural, people don't even notice. And even if you avoid designing personality into your system, people ascribe it. So you might as well take advantage of it. And you can make a system seem conversational even with a constrained recognizer instead of a grammar-based one. There's also the issue of people responding in human ways to the computer.