It is not that but i have looked at all kinds of books in the book store in which case i stayed so long they thought i might try to steal a book or two.
I have visited my share of sites that sell books. And spent hours taking notes on books sugested and checking the review of these books and then i verify with members here and cross check.
Take for example traps that are well known in openings or not so well known. They will not be found in FCO for the most part. I had to find these things by exiperiences players at chess.com, Vote Chess and master games. All which helped my game improve faster than my books. Which lead to me buying a book on Opening Traps.
I found lines of play could from players at chess.com i did not find in my books. Immediately i started winning games with those examples which i can not say about the books as it took longer.
FCO sounds like a great reference book. But when you start playing more games at chess.com your opponent has a chance to look at your games and become famaliar with the lines you will use. Then they go out to the public forums and ask how to beat that opening(Which it seems like it is an infinite amount of threads how to beat several openings or help beat them /make it difficult for the user of the opening). I say once you have 500+ games of a certain opening you start to see various variations that you never saw before unless your a titled player or 1900+.
I have been playing a couple openings for over 2 years. And for the most part i see something new about once a week and after the game when i went to research it i could not find it at first. But found it similiar to some other opening which gave me comfort zone on how to maximize my attack chances against it.
I am currently studing 2 openings seriously. My book just show 1 line. Well that one line is very much explored here at chess.com and very good. It has several variations that make it tought for my opponents. Now when i consulted with some other players and their rank was lower than mine. But i am the beginer and from my perspective they are the expert. They know what works and have experience with it so i ask them to save time studing it also. They showed me some tricks which may have taken me days to study to consider and determine if worthy to use in a game. I say in 2 days i must have recieved 2 weeks of info on those openings if i did it by reading the book. I figure by another month i will as comfortable as i am with any opening i am currently using in a game.
I like MCO--it lists all of the major openings and explains the basic ideas of each opening.
You're probably beyond even needing an opening encyclopedia, but if you seriously think MCO explains the ideas behind the openings, you really owe it to yourself to at least check out a copy of FCO.