title: What is the main reason to use new grid system?
body: <p>I want to migrate my project from TBS 2.3 to 3 and I've got a question:<br>
  Why should I use new classes like col-*<em>-</em> instead of span*?<br>
  Since I can use both of this files, new bootstrap.css and old bootstrap-responsive.css on my   page and old span* do their work well...<br>
  any suggestions?    </p>