Good UI design is all about effective communication. If you have a UI that is pretending to be something it isn’t (a person) then it is basically lying to you. That’s hardly good communication. It’s confusing and dishonest.

And believe it or not there are some users who won’t understand that it isn’t really a person. Some minds are very prone to anthropomorphizing especially when they are dealing with something they find hard to understand and unpredictable like a computer.