It also removes a lot of the value of the LLM. They're perceived as being smart, and the interface (open-ended text) implies they are capable of more than executing pre-defined functions.
So if you have a "CalculateQuarterRevenue(year, quarter)" function, you'll soon find your users asking for the data per-month. Or just for the last six weeks. Or just for a specific client. And they'll be confused when it doesn't work.
The conversational user interface is misleading then, isn't it? It can't make you a sandwich either, though it allows you to submit this request.
Yes. The sandwich example is contrived, but the basis is "discoverability." It's very opaque to the user what actually can be done and how reliable the result is.
Compare this to basically any website you've ever been to. It's the "GUIs vs. CLIs" discussion all over again, except even CLIs had man pages for discoverability.