After nearly 20 years hiring in the workforce, I would never suggest to anyone that a cover letter is optional, and the subsequent points you make are exactly why. The resume is "what" and your letter is "who."

Another important thing — people spend a lot of time on their resumes and far less on their letters. Though different documents, they're part of the same utility, so the same level of care, proofreading, spell-checking, and creativity should be applied.

If your resume is well-crafted and spotless but your letter is riddled with errors — or worse, a completely different tone — that's going to speak volumes.