Letters of recommendation are typically used as a means of assessing an applicants qualifications, capabilities, and characteristics in terms of ability to perform a specific task, in this case succeed in medical school. Which is why LoRs from individuals with titles (PhD, MD, DO, etc.) are always preferred in the case of medical school applications. And while you could technically consider them to be "elective" letters, keep in mind that requisites are simply that and that sometimes that's not enough. Additionally, you have your personal statement, supplementary essays, and extracurriculars to detail who you are, you don't need someone else to do so.