Good one...
My personal experience tells me that I never hire people who don't write a proper cover letter. Some will write "Dear HR," some will say "Hi HR," some will say "Hi," and some will just attach their CV with a message "PFA my CV." Others will write in horrible English with spelling mistakes or use SMS language, which does not encourage me to open the attachment.
Once, I informed a candidate three times that I would not open their CV until they corrected their cover letter. Despite marking all the mistakes in yellow and reminding them, by the third reply, they still hadn't made the corrections. Consequently, I had to ignore that candidate.
Thanks,
Geeta