"The secret life of JSP" doesn't really discuss why using a JSP for processing is not a good idea.
However the second linked article :
The Front Man does. I suggest you go read that one to find the answer to your question.
Here is one reason given:
Not only was the code hard to read, embedded as it was amidst tons of Javascript and HTML markup, it was untestable, unable to be reused, difficult to refactor, and just a plain bear (in a bad way) to deal with.
and what is exactly mean of the line: "that all template text in a JSP including white-space is emitted as part of the document. "
Pretty much what it says.
You include a carriage return in a JSP, it carries that carriage return through to the generated HTML.
Yes, Servlet data is part of the document. The difference in a servlet is that it only outputs what you tell it to via out.println() statements, so you have finer control.
JSP can add extra whitespace - which for the most part when dealing with HTML doesn't matter in the slightest.
You should just be aware that it does for those 0.01% of cases where it DOES matter.