I think David was an interesting character in an interesting predicament. He could easily have disastrous effects for the Animorphs during the war, all he'd have to do is tip the right person about who they are, and the Human resistance is easily destroyed just like that. As a rat nothlit, there's no chance of a Yeerk even wanting to infesting him, and his parents are already controllers, so what would he have to lose?
If David had gotten a deal similar to Tobais, in that he'd be a rat nothlit that can morph, that could be potentially thrilling to have him as a recurring villain. But at the same time, the Animorphs could so easily be defeated, especially during the events of books 20-22! But he didn't, and decided instead to plan with Crayak to get his own personal revenge, specifically on Rachel.
Also, I wish at the very least they could have addressed to David's parents about what happened to him, after the war. Even just to lie and tell them about how he "fought bravely and gave his life," or something like that.