Your cover letter is not nearly as important as your resume, and it may very well be overlooked in the recruitment process. This is far more common for large firms with many applicants, but may still happen at a smaller firm. Despite this, while a good cover letter may not help your chances, having an average, bad, or, worse, no cover letter could really hurt your prospects. You never know when an analyst is going to get bored and look for something to amuse himself. Everything you do in trying to obtain a finance job should be done to the best of your ability. Anything that is less than perfect is, frankly, a potential reason to ding you.
A well-written cover letter should never take up more than one page, and it should be three to four paragraphs long. The key here is not to try and tell your life story or focus on anything other than the firm, your interest in the firm and your qualifications for the position you are applying. Check with your career services office for an example off a strong cover letter, and make sure it was made for the industry you're interested in. When you are ready to send it off to firms, make sure to take advantage of WSO’s Cover Letter Revision Service. This service is a fantastic way to have someone who works in the industry and goes through applicant's cover letters review what you are sending out to make sure it is acceptable.