Here's the real deal. If you don't do any prep at all you are interviewing with a ton of people who have. So you are being compared to other candidates who are more polished and practiced than you. None of the reputable prep companies "can" your answers. And if you haven't practiced the LOI stuff your going to look like a dingus next to someone who has. It's an unfair world we live in that you have to pay people to help you get these jobs. But it's just the way it is. And if you hire someone who is selling you right answers then fire them. Can you get hired without it? Absolutely. But seriously why take that chance? Just my 2 cents. During my interviews (3 of them) just the confidence in having told my stories to someone who was a professional made me feel better.