I'd argue, but its one of those things where I'm pretty sure I responded to it as I did because I "felt" the characters, particularly Naota, which is one of those things you need a certain mindset for. At the time, I was 16, and only just starting to become aware of myself as "an adult" as it were.
I think I accepted robots growing out of his head and the like because I felt it was its own reward, and didn't concern myself with an explanation, probably because I had already been won over by that point, and took it in good faith that it would be satisfying.
As for Mamimi's pyromania, that never really went anywhere on its own, and I would concede to an accusation that it recieved too much attention, or maybe even wasn't necessary, but its purpose was to push Mamimi's behaviour out of what could reasonably be described as "normal" in order to reinforce her self destructive lifestyle, as well as demonstrate that it's becoming external.
Somebody track down Bubblecat and show him all this premium discussion.