This has quite obvious evolutionary advantages. If you can predict how someone feels or what he might think you can outsmart him in competition for food. You could trick him for trade (e.g. food for tools) when he least suspects that it might not be in his favor. Essentially externalizing mirror neuron processes is much more beneficial in the short term, if you can think how the other person might think. In the long run in order to be successful you also need to internalize you mirror neurons - you need to think how to think about yourself and to start thinking what this person might think about you and what kind of self awareness they could have. You can't just trick the other, the deal is to trick them without them noticing otherwise you end up with prisoners dilemma. Probably those processes could also had led to evolution of language and trade.