That's hard to argue that with upcoming shift to Drupal 8, developers are going to face a lot of difficulties getting used to a brand-new architecture of upcoming Drupal version. While in Drupal 6 & 7 use of advanced IDE is recommended (but in general you could live without using one), version 8 sets a new level of challenges with Symfony2 components included, Twig as the main template engine, OOP introduced, and sophisticated hook & API system in place as well.
And now just imagine that your IDE deeply understands your Drupal code providing context-dependent code completion, safe refactorings, respects PHPDoc and integrates debugger, PHPUnit and many other tools. With this deep understanding, you can use your tool to navigate between hooks implementations and invocations, Drupal API & core methods, and instantly get quick documentation for everything you are working on! Adding to that built-in Drupal Coding Standard (so that you can automatically re-format code before commit), Drush tool which is run right from the IDE, and much more.
During this session we will be working with PhpStorm IDE which has been quite a gamechanging and trending development tool in Drupal community lately.