Cover letters that just say how much you love the company are pretty useless. Cover letters that provide additional information about why you're a good use of the company's interviewer's time are useful. I've submitted letters about, say, how I think one project on my resume is specifically applicable to a problem the company has. This both shows I researched the company, which is proof of investment from me, and provides some evidence that I should work at this particular company (or I could have done bad research and misunderstood the company and what it wants, in which case it will help them reject me and help me find my way to a company I'm more interested in)