WCS is a management software used for managing network comprised of controllers and Light Weight access point. In short its the management system for Cisco's unified solution. WCS is of no use without controllers. But controllers can be used without WCS but it will be difficult to manage if the number of controller increases.
You don't need a WCS server however you will require WLC's to control your Wireless WAP's. The WCS allows you to add multiple WLC's into one view along with floor plans and floor layouts and alerts. Without a WCS server you would have to manually login to each WLC controller to see what the WAP's are doing. It really depends on how big your site is and how many WAP's you have to deploy after a site survey is done.