These letters are important cant advise on visit visas as never applied for one. But for a spouse visa the cover letter as i knew it which i addressed to the ECO basically went over all the points i thought the eco would need to grant the Wife a Visa. I.e we would not be a burden on public funds, that we would be self sufficent more detail about my accomdation etc.
To me it should make the ECO's task basically a formality as all his or her questions are answered in a single document.
