title: Singleton pattern in all classes
body: <p>I have very quick question.
  I'm discussing with my friend about Singleton pattern.
  They say it's not good to use in all classes, and I have a different opinion.
  Is this true and why it is important not to use it in all classes?
  It makes no sense to me. Pattern is used to prevent multiple instantiation and that is it. What's the problem?</p>