Apparently there are resource and damage reduction penalties applied to our skills if we use them through the hotbar instead of through the keystroke combos.
http://forum.blackdesertonline.com/index.php?/topic/15423-hotbar-skills-using-more-resources/
Of course some of the penalties discussed are hidden and hard to test.
This is the weirdest penalty system ever. If they don't want us to use a skillbar, they shouldn't put one on the UI.