They certainly should be. I know they're introducing new characters like Vision, Black Panther and Doctor Strange, but we NEED The Defenders in the MCU- actually, they are in the MCU. The Netflix shows Daredevil and Jessica Jones as well as the two upcoming ones of Luke Cage and Ion fist are actually set in the same universe as the rest of the MCU, at least that is what I heard. Nevertheless, they need to show up in the movies!!!
So far, in the TV shows that have been started (Daredevil, Jessica Jones) there have been no contradictions to the MCU containing the big stars (i.E. Iron Man, Cap, Ant Man, Hulk, Thor, etc.). In fact the Netflix original series have done just the opposite! Throughout both series the battle of New York that took place in the first Avengers movie was mentioned and had a large impact on the plots. It should be fairly easy for the movies to include these heroes in the Infinity war movies coming up.
We saw the problems that came with Age of Ultron. There was too much content trying to be introduced in that film that you couldn't possibly create a decent story.
In addition, it comes as a great benefit to keep the defenders in their own netflix series. They have more freedoms and can take on a more serious tone. They can develop their characters far more and explore an entire world the Avengers don't occupy.
I love everything about the MCU, but It's so beg that it needs these separate mediums to keep everything nice and in shape.
I wouldn't mind seeing a cameo or some interaction though.