The point of having heroes fight it out on screen typically comes down to it being a simple way to define:
1) Their strengths and powers in comparison to one another
2) Their goals/the conflict which will push the character drama.
We know that Cap is super strong, but he couldn't take down Loki. Stark has better luck. Oh but Iron Man's armor can be crushed single handily by Thor, and the only things that seem capable of surviving a direct swing from that thing are Hulk and Cap's Shield. Oh look, now we know were everyone is in the line up and we didn't even have to all sit down together and discuss stats.
So whether the heroes fight or not will likely come down to those two factors: power level exposition and drama. I think a lot of the first part comes down to what they do on Luke Cage and Iron Fist and if the expectancy for The Defenders will be that the majority of the audience saw at least two of the shows, or if they will approach it more like Avengers with the idea that the team up will bring in a bunch of new viewers.
As for the drama, it is almost always used as short hand to show that these two character's personalities will be at odds. It's shorthand because instead of having the two characters meet and the conflicting elements of their personality or goals becoming apparent through more normal socialization you just have them mistakenly fight each other and BAM we now instantly know that these two people clash. But by the end Luke will have (likely) met Jess, Danny, and Misty with the last two also knowing Coleen. Any immediate clashing will have already taken place, and even with characters like Jess and Coleen it's like... Wait, where are Danny, Luke, and Misty and did none of them bother to mention to their friend that there is a drunken superhero and redheaded samurai running around the area?
But there is a difference between fighting against and fighting with someone and I fully expect the last to occur plenty. Even if you take six perfectly nice people if you put them in a stressful enough situation eventually you get some conflict. However, as far as we know it's NOT six perfectly nice people. As of right now it's three possibly reasonably nice ones, two disgruntled loners, and one guy who seems to purposefully distant himself - a trait probably not at all helped by what was done to him by certain Jessicas. Although at least Luke has shown some interest in playing ball. Both Jess and Matt have been very instant about that whole "I must be an island. And I mean one of those futuristic, floating ones that touches absolutely nothing, not even the water in case that somehow connects me to other people" and while Jess's series ended with her becoming somewhat more comfortable admitting that she had people she cared about, Matt's put him in the opposite corner. So yeah I'm sure those guys are all gonna be just perky about a team up.
