I know php/html/js/jquery. Should I learn a CMS?

At my day job, I created and maintain web application that does shop floor management, shipping & receiving, inventory, EDI, HR, and other little things. It is written with html/php/mysql/jquery/bootstrap. I include() the css, scripts, and page header for every page kind of like how a CMS does. My question is should I take the time to learn a CMS for side jobs? I am playing with Drupal and Modx but working with the database is a pain (I know it's because I don't know the API well). With php I can have a script wrote in 10 seconds. I would imagine that a lot of freelance work wouldn't involve a database that much. On my GoDaddy host I just clicked a couple times and Drupal was running. So, is the time saver just how quick you can install a site on a hosted server?