Why are cover letters limited in value? Three reasons. First, most people assume that the cover letter is actually read before the resume. Wrong. Just ask those who spend any portion of the work day reviewing resumes—they typically go past the cover letter directly to the resume and only look at the cover letter if they are still interested after their initial resume review. In my review of more than sixty thousand resumes over the past twenty plus years, I have probably read only ten thousand cover letters. It is actually rather amusing to watch a Hiring Manager reading a newly arrived resume. The cover letter is put to the side, and the resume is scanned first, then read. And you know there is interest if they finally make their way back to the cover letter.
Second, most people assume that the cover letter should be about you. Wrong again. It should be about the company, your prospect, your target. Your resume will tell them everything they need to know about you (if it is well written). If you are interested enough in the company to make an initial contact, take the time to fully reflect your understanding of the company and how you may be able to meet their needs in your cover letter.
Third, and most important, many college students end up using the cover letter/resume mass mailing/e-mailing/posting as a crutch to convince themselves that they are actually doing something in their job search. "But I sent out over two hundred resumes!" In reality, all they are doing is generating rejection letters. Mass mailing/e-mailing/posting of your cover letter and resume has extremely low odds for success in today's job market.
Please understand that at the entry level a resume and cover letter on their own do little good. Most larger companies have established college recruiting programs which serve as the focal point of entry level hiring. Therefore, unsolicited entry level resumes are often ignored. Many small and medium-sized companies do not have the internal resources necessary to train entry level hires, so the entry level resumes are simply filed. The best you can hope for in a blind mailing campaign is that you will be filed away and perhaps miraculously resurrected at some future date. Highly unlikely.