This has always been such a fascinating issue, especially in the aviation industry. I am able to hire a resume/cover letter assistance company, forward all of my credentials on to them, and they will respond with a traditionally formatted resume and a cover letter that differentiates me from, say, Joe Shmoe, in only a miniscule way. If Joe and I have similar credentials, experience, etc... a cover letter then becomes just another piece of paper saying very little. It's like printing money with no tangible backing - it's just another piece of colorful paper. I think these services are excellent, certainly for those who are not as fond of creating and writing, or those who are not terribly computer literate, but perhaps not necessary for all.
When I worked as a manager tasked with hiring, it was very easy to weed out the dopes and idiots - you just look at how they spell, present themselves on paper, proofread their work, etc...
For Frontier (and all may mock me if they wish), I included a cover letter which, toward the conclusion, mentioned that as a father to two boys and as a spouse, I have learned negotiation skills as well as utilizing the resources available to the crew consisting of my wife and myself. I went out on a limb, though, and briefly explained how our four year old loves the animals depicted on the aircraft tails and hence believes that many of the Frontier employees may, in fact, consist of animals. My son is convinced that my cover letter should therefore be addressed to the spokes animals. So I took his advice. I wrote three cover letters to three of the more prominent spokes animals for Frontier as well, complete with fantastic animal puns, metaphors, etc... I figured if it makes one person chuckle on the other end of the email address, perhaps, while they may mock me behind my back, they may also find that simple cover letters and resumes crammed with airplane numbers and tallies presents very little of the applicant's personality. If it doesn't come to be, then so be it. I'll just ground my four year old, send him to his room and tell him not to come out until he finds another clever way to get daddy a job...
As of yet, no plans for streamers and underwear at the interview. That will come after the offer letter...
Cheers!