I don't personally believe a cover letter (or resume, for that matter) is necessary.
While none of us know exactly how Disney Recruiting chooses which applications go forward in the process, the most current and commonly accepted theory is that additional documents, including cover letters and resumes, are not opened or considered at any point during the DCP process. Many people go so far as to believe that there are so many applicants, and so many spots that need to be filled in such a short amount of time, that Disney employs the use of a computer system that electronically scans the applications. It is commonly believed that by electronically reviewing the applications alone, where you will manually input your work experience and skills, the program chooses which candidates to push forward in the process. The recruiters simply don't have the time to read almost 10,000 cover letters a season.
FWiW, I didn't upload a cover letter or resume when I applied and was accepted, Fall 2016.