I thought Jazzy's first post was very helpful and that she (or he-- sorry I can't tell from you profile) was very clear both times. It's sort of, well, rude to compare how helpful people are, if they're all being helpful. There's a way to compliment someone without taking a jab at someone else. All of this info is volunteered. No one's obligated to answer questions, not even follow up ones.
As for the cover letter, and obviously this is just my opinion... There are plenty of people who got into 4 or more top ranked programs who didn't include a cover letter, myself included. I'm sure the cover letter is not a make or break thing -- or even very helpful, by most accounts. At best it's overlooked or ignored in favor of the SOP/WS; at worst it annoys someone on the committee because it wasnt asked for, and why should one applicant get more time and space to "make their case," so to speak? (Not my opinion - just playing devil's advocate here!) So the net effect is either 0, marginally positive (but not enough to make a difference, all other things being equal), or negative.
All of this is to say that the time is probably better spent on the SOP and writing sample. Unless you can dash off a quick cover letter! I am being 100% serious when I say that at every school, THAT's what they mentioned post-acceptance. They quote things, point out what they liked, what they could incorporate in their own work, how their school's offerings align with my interests, etc.
It's ridiculous that so many great applicants don't get admitted anywhere. So the other thing I will add is that luck was a big factor. You can do everything right and still not get into the school where your work and background "fit." Those are just the odds. I totally understand the point of this thread and it's definitely helpful! But I just want to say that it's also a crapshoot. I don't know what I did to get in, not really. I don't know why they picked me over someone else who is just as qualified, who fits just as well. I can guess. But none of us can know for sure. And in a pool of highly qualified people, the power of luck shouldn't be ignored.