As both a developer of multiple websites I like the following advantages that are available with Joomla (when compared with Wordpress)

• An well thought of User/Access management system
• During the maintenance phase having better control allows you to expose features to your clients especially when they tend to get overwhelmed by options
• When required I can place similar restrictions with my team
• Easy & well-defined ways for overrides to prevent changes being lost during updates
• Module positions in templates gives a lot of flexibility when sections of the website have slightly different layouts
• The above benefits basically make it much more comfortable to have a more future proof site that can adapt to changing/evloving client requirements