A good way to see this problem in action is to compare Siri and Google Now, both of which are of course bots avant la lettre. Google Now is push-based – it only says anything if it thinks it has something for you. In contrast, Siri has to cope with being asked anything, and of course it can’t always understand. Google Now covers the gaps by keeping quiet, whereas Siri covers them with canned jokes, or by giving you lists of what you can ask. The actual intelligence might (for the sake of argument) be identical, but you see Siri failing.
To invert this, the Siri ‘this is what you can ask’ screen is essentially a command line help prompt, whereas the whole point of a GUI was that you didn’t have to know what you could type anymore. Though natural language processing means you don’t need to know a specific syntax for Siri or a bot, there’s still a basic discovery issue – what can I ask, given I can’t ask anything?
All of this means that for now, it seems that a bot or conversational UI might work best for something very specific – where the user knows what they can ask, and where those are the only things that they will ask. However, when it does work, it becomes very interesting indeed, particularly now because it happens to align pretty well with the second preoccupation – getting around the app-installation problem.