It's the developers of agents like Siri and Cortana, not the users, who need formal language.
Anyway I don't think we're disagreeing with each other: there is always going to be a place for formal languages, they won't ever go away and be "replaced" with natural language interfaces, it's just that Jane Random will be more than happy to speak some gibberish to her computer and let the machine figure things out.
Oh but we are: I don't think Jane Random will ever reach so far into that direction until her desires become vastly simplified. So in a Wall-e style future then yes, but in one where people still do things creatively there will always be need for formality.
Anyway I don't think we're disagreeing with each other: there is always going to be a place for formal languages, they won't ever go away and be "replaced" with natural language interfaces, it's just that Jane Random will be more than happy to speak some gibberish to her computer and let the machine figure things out.