I am going to apply for MBA programs, maybe not for this year but ones that start in 2015. I am from Germany, but my goal is to do an MBA program in either the US or in Canada.

How important is the TOEFL? I have limited capacity to prepare for standardize tests, and most of my energy will be on the GMAT. But for example I saw that NYU Stern requests candidates who haven't done a degree in an English-speaking country to submit a TOEFL score. Is this common?

Of course: applicants aiming to study in a foreign language always need to prove their foreign language skills.

The main purpose of TOEFL is to improve your language skills and i don't think MBA Colleges take TOEFL Score

You are mistaken. Universities require that students prove that they can speak the language in which they will be taught. The TOEFL and IELTS are the most common way for students who speak English as an additional language to prove their ability.