Company I worked for would ask for a cover letter. They would rarely read them but if you didn't do one, your app would be tossed in the trash can. It was amazing how many people would not attach a cover letter. We came to the conclusion that this was because there are people out there that don't have the skills to actually write something. This is even more surprising as there are hundreds of cover letter templates on the web.
Yes on the rare occasion they would read them, people would forget to change the company name in the letter.