That's a good point. It's one Poparena takes note of in his reviews. A lot of fans think that the Animorphs should have left Erek with the Chee. But one could argue that there was just too much going on at the time--the Summit meeting, David finding the box-- that the Animorphs sort of cracked under pressure and made some bad decisions. The Animorphs making bad decisions isn't at all detrimental to the story, though, as it shows how they have grown in how they handle certain decisions. The David Trilogy is considered to be a "turning point" of sorts for the series; it gets notably darker as the books go on from there. That could be why David was so poorly handled.
Also, I think K.A. just wanted to give more reasons as to why David was such an unsympathetic ****.
Another thing I've been pondering--though, it's probably a stretch:
Why didn't Jake use one of the Chee to impersonate Tom while the Animorphs starved his Yeerk?
Again, for the sake of the plot, I see why they didn't do this. But I'm sure that Jake spent many a day thinking of ways to free Tom, so maybe something like this crossed his mind? He could have asked any one of the Chee to project an illusion of Tom, go to the Yeerk Pool every three days to "feed" while Jake and the Animorphs keep Tom in the Hork-Bajir valley--or, perhaps less of a risk, the shed where they kept Jake-- and starved the Yeerk? They could have used that homeless Chee from the Exposed to pretend to be Tom for when ever Tom was supposed to feed. Tom could probably live at home, as though nothing happened, and supply the Animorphs with a wealth of information. The only problem here would be that he would have to keep up the charade when confronted by Chapman, and other controllers. And, of course, he'd still have to attend the Sharing. But they could always just keep the free Tom in the Hork-Bajir valley while a Chee lived his ordinary life. It would be a hell of a lot better than being a slave.
Again, it's a stretch. I don't think the series would have been as great if they had gone this route. But it's neat to think about. I can see why K.A. wouldn't want to overuse the Chee-- the same reason she doesn't use the Ellimist all the time: too over powered.
Hell, things for the Animorphs would have been so much simpler if they just had the Chee go to school for them, while they caught up on some z's in the Hork-Bajir valley. I mean, seriously! That's the perfect place to do Animorphs things! Why stick around in a barn, risking possible interruption from Cassie's parents, when they have a Hork-Bajir valley that only they can see! Come on, Animorphs. You've got these sources at your disposal. Use them!