not even close They are hardly ever read; nobody has the time to open two documents for each application!
However, the majority of individuals misinterpret the intent of a cover letter. Its primary purpose is to encourage the recipient to read your resume; it is NOT an addition.
Therefore, what should a cover letter contain? Take one or two major needs from the job description and briefly - and I mean briefly! - describe where and when you met those requirements and quantify or define your level of achievement.
Should not exceed three brief paragraphs.