Except that it's already there. She's not 'seeking it out' or 'inviting it in' or anything like that. It's there, and pretending it's not isn't going to help. That much is obvious. The thing is, right now we've got a situation Shiki doesn't know enough about. She doesn't understand what's going on. So why not force the voice into a situation she does understand? If the voice refuses to proove what it says (which, remember, is the expected outcome), that's concrete evidence of deciet. Also, remember that so far, the parser has been rejecting commands that Shiki's current self-confidence issues would interfere with. The main point of this would be to start countering that problem.
And also, remember that Shiki's assuming, and in fact counting on, the voice acting in accordance to its decietful nature. Do you really think exposing deciet, especially in a situation like this, is against Shiki's character?