I went through the ECIC prep over a year ago and thought it was beneficial in getting in the mindset of answering TMAAT questions and finding out my strengths and weaknesses. Ie I was sounding rehearsed so I worked on being more conversational with relating my stories and experiences. Ie practice as if you were talking to a friend of your mom that has no idea about aviation. Was on of the examples used to help me. I was using too much military jargon! Lol. I don't think prep is 100% essential, but when competing for this job would you rather have done everything possible to prepare? All my friends that have been hired at the majors have done interview prep FWIW. The other American Airlines hiring thread has a wealth of info in it.