I will now explain the way you should learn the ECO codes if you don't know them. 
  
First learn the first number for each code and all the moves that cause it using this page: 

http://scid.sourceforge.net/help/ECO.html

Make sure you know them, run them through your head a few times on lots of occasions and you'll know them all for sure. Also have an example for some of them. For example: B21 - Smith-Morra/Grand Prix. B33 Sveshnikov.  
 
Then you can continue to learn the second number of your favourite openings or openings you see a lot online. Like B21 and C54, C57 and B85. But you will never make a mistake on the first number if you learn it like this.  

Do it this way and it's very easy, kind of interesting, and IMO worth it. For example, if you "his" some handle on fics, you can see if they're a Dutch freak (A8-A9), play various openings, are a B07 type of guy or a C95 one, and just get an impression of their play overall. It's also REALLY useful for searching openings, and is the fastest way to read what opening has been played. So it's definitely worth learning.   

Also: "how big the c3 Sicilian is now"... it still has only one page in MCO-14. It may have lots of dedicated material, that doesn't make it a very critical or extremely relevant line for most people. Worse is probably putting the entire 1. d4 .... 3. Nf3 in E10-E19 while Nc3 and g3 get the whole of E20-E99.