He's my favorite in the UFC for a variety of reasons, but biggest is that he backs up his talk with results.
The trash talking serves two purposes in my mind:
1. Self promotion. Floyd Mayweather was a genius in this regard and you either loved him or hated him, same with McGregor. But the results are the same, people either REALLY want to see you win or REALLY want to see you lose, boosting ticket sales and hype.
2. Gets in people's heads on an emotional level. I respect Aldo and think he's a great fighter, but he should have ignored McGregor and written him off. The guy had too much pride and he allowed McGregor to hurt it. Therefore, his expectations going into the fight were heightened. He was defending his title, which already puts the pressure on you. No need to take McGregor personally.
Will he beat Rafael? I think many people are betting against McGregor, Rafael will be his toughest opponent to date and out classes McGregor in size alone. I think where McGregor beats Rafael is he has a high fighting IQ that allows him to make smart decisions and adjust mid fight. If all things are equal (height, reach, size, strength, etc) McGregor wins out alone in mental toughness and fighter IQ. I think he loses, but I would not be remotely surprised if he wins. And if he does, I'll never doubt the man again.