Villagers, my favorite entity in Minecraft, are now being discussed again :D If you haven't read my two theories about Villagers, I recommend you go through them now for a deeper understanding in this one. Here they are:
-http://www.planetminecraft.com/blog/minecraft-theory-villagers-are-not-what-we-always-expected/
-http://www.planetminecraft.com/blog/minecraft-theory-the-truth-about-chiseled-sandstones/

Without further ado, let's get started! As I believe, Villagers had once been adventurous and brave, but now they are the opposite...Why? Since players came to life, they destroyed and stole everything, made Endermen close the portal to their cities. When they couldn't go to the End directly anymore, they started to get angry and put their hatred on villagers because they were friends with Endermen and many other creatures (In my theories mentioned above). On the other hand, Villagers, was able to fight back...however, villagerforce couldn't overcome playerforce due to their lack of population.

The first part I discussed doesn't have anything to do with the topic, am I off track? Nope, that part will answer why Villagers like Emeralds so much. As they couldn't win the battle, they played it smart: to become friendly, passive and most of all, make emeralds trades. How come this is important? Well, emeralds are useless (unless for decorating), so Villagers began telling players to trade them Emeralds for something return. Players, they are not stupid enough to deny that because finally, they found a use for Emeralds, is to trade with villagers for better stuffs. Therefore, Villagers will unlikely to be killed because if they were, players won't be able to spend their emeralds. That's is the reason why Villagers make Emeralds trades: to keep them alive. Just imagine, if no trades were made, it would be hard to find a village that has any residents in it XD ('cause they were all slaughtered).

However, to prevent any fatalities, they also make Iron Golem, to protect them if players still attack the village.

That is it guys, now you know why Emeralds are favorited by Villagers :D Hope you enjoy and debunk freely :)