I did a presentation on "dress" with both ends... business casual and business formal (coat and tie or suit). In each case, the "suit" was deemed more believeable, reliable, accurate, trustworthy, and convincing. Even when the casual was the expert.
And as Andrew stated, the tie allows him to bill and extra $20/hr. With the tie, you're professional. Without you're just "good".
I only were ties when I am going to be making a senior management presentation.