Its true that they do ask questions outside the syllabus in JEE. But I think its also true that many students never check the syllabus and read too much.
In recent years, I have seen that so many questions are from syllabus specific. If I remember correctly, Organic syllabus given is very precise and in exam also questions are from there itself.
So focus more on the topics given in syllabus. Dont ignore the syllabus. At the same time, if you think you have the potential for a great rank, there is no harm in doing your coaching material.
Ultimately Jee Advanced is an exam where you expect the unexpected. So even if we say something.. we can never be sure. And hence students cover their coaching manuals just to feel secure.