I came here because we are currently re-watching the series and are at the point where J.R. and Bobby are fighting for control of Ewing Oil as per Jock's will.
And then noticed that many of you mentioned Mickey, and I agree with you. He had a great storyline-- a fill-in son for Ray, and what looked like a future husband for Lucy. He was underutilized because many of us wanted to see the relationship between Ray and Mickey grow, and also we wanted to see Lucy happy for at least a few more episodes.
The fact is that Mickey had to die.
The writers wanted to show how the battle for Ewing Oil would seriously hurt, if not destroy, the lives of many people not involved in the battle, including Lucy, Ray, and Donna.
How could the battle hurt Lucy, Ray, and Donna? By having Mickey come along. He is a fill-in son for Ray. And we can even imagine that he and Donna will come to appreciate each other and he can be the son that Donna never had. They begin to show how Ray is to Mickey the way that Jock was to Ray.
Then, Lucy falls in love with Mickey. It is easy to see that he is the first boyfriend who truly made her happy. We could even imagine them getting married.
When Mickey is killed as a direct result of the battle for Ewing Oil, it showed that the battle deeply affected Ray, Donna, and Lucy.
To put a finer point on it, without Mickey those three would have been unaffected by the battle, and the writers (painful as it was) wanted them to be casualties of the battle, right along with Walt Driscol, Rebecca Wentworth, and many other innocents.