Rakell, I agree with you that such advice not to use contractions is probably a wise one. But my reason would be that non-natives are likely to misuse contractions, not knowing when and where to use them. Here's an illustration from a research paper from http://www.brookings.edu/research/paper ... mBrookings
"It is clear that different segments of the high school population need different postsecondary opportunities. Some are academically able and should be applying to selective schools. Others are much less well prepared and might benefit more from a one-year certificate in a high-demand field such as health, computers, or welding. One size doesn’t fit all."
She opens with "It is clear", and doesn't contract it. She ends with "one size doesn't fit all", a contraction. It's a stylistic device. It works when you do it that way, in a concluding sentence that highlights the point being made.
I know I can find many other examples that contradict this so-called rule. At the same time I can see why it might be taught to learners of English as a commandment.