If the focus is on highlighting how your relevant skills can help the employer then you may have to adjust the CV every time you apply for a role. I suppose my advice is geared more towards getting your foot in the door. There is absolutely nothing wrong with following your CV with an email listing your relevant skills and how they may help. The issue with cover letters is that most people write exactly that, a letter. Employers don't want to read a story, they want concise, easy to read facts that you can substantiate.