Over the past two years, booking three trips for my wife to Europe (summer 2011, 2012, 2013) I made a lot of mistakes, and did a lot of learning. I mostly know what I'm doing now, but it was a long slow error-filled process.
I think I went from where I knew 20% of what there is to know, to probably 80%. My wife asked me to show her sometime what I learned, and I realized I could, but it would be so filled with exceptions and one-off knowledge (Lufthansa does not release F space to partners until 2 weeks, Tarom doesn't post availability until 9-10 months ahead, US shows no partner space on their website, DL shows only AF and KL, United shows all, BA has huge surcharges through LHR but AA doesn't) that I really had invested a lot of time and effort. I'm pretty comfortable that the booking services earn their pay, and that a LOT of people would be much happier leveraging their knowledge than trying to acquire it on their own.