Javascript and PDF

Have a look at Google's answer when both "PDF" and "Javascript" are in the search box. When I did, I got 4 results out of 10 concerned with security faults.
So, here is my initial question: Why should Javascript be put inside PDF files?
Answer: it's in the ISO norm defining PDF 1.7, with no precise details, but at least references to more detailed documents.

It's long known to web developers that Javascript is a nest for problems, especially when it's not correctly documented. Yet Adobe looks to develop forward the possibilities of its software, its file formats and that's normal. However I would wish they did it differently. First, that they did not melt innovations under a unique "PDF" name, which refers to a format that users choose primarily because it's supposed to be portable, simple and solid like rock. Then, that they did not activate Javascript by default. Few users really require it and even they recommend to deactivate it.