I think coders such as Pescado dislike hacks such as InTeen because they are so huge and far reaching, that almost every other mod written tends to create conflicts.
While, not being a coder myself (so forgive my invented terminology) , I get the feeling that JM likes small(ish) self contained code that has a tight focus and dislikes large mod that includes everything but the kitchen sink.
And from that point of view, I understand how irritating it is to have people come here and complain his hacks not working because of a conflict with one of those big mods. Bad enough having to work around Maxis bad coding, without having to figure out everyone' else as well. (not implying that their code is bad, just that it takes time and energy to figure someone else code).
Anyway, that's my take on it. I could be completely wrong, not being a modder myself.
Being a coder myself -- albeit one who has never delved into the mysteries of hacking "The Sims 2" -- I would say you've accurately summed up what would be a frustration for me.
That being said, I do actually like the InSim for very specific uses. Particularly when I'm creating/customizing my own townies. The alternative would be bouncing back and forth between the game and SimPE more times than I care for sitting through the game loading up.