I think you are confusing mentorship with advocacy and sponsorship.
When you say your "PRF looked pretty good"...I hope you mean because you had a record of superb performance, stratifications, contribution...not that actual way the PRF was written. Your tone seems to indicate otherwise, as in your mentor showed you how to write a grammatically perfect draft for your supervisor.
I am not trying to be an asshole and yes I get it, there are still some lazy supervisors out there who take a "draft" OPR or PRF and push it forward, but the rules have become so extreme with regard to how things are worded, quoted, documented...this is really commander business and has zero to do with mentoring. Yes your mentor may help you work through goals and important boxes that must be checked for promotion, but there is FAR more to it.
My concept of mentoring and what I tried to do for many people over many years was provide LIFE and Career advice that often included a dose of painful feedback. Mentoring is often more about listening than speaking and while the advice may focus on building a successful career, it should not be the only intent. I've mentored good folks through very bad times including divorce, a death in the family, PTSD and just dealing with the normal stresses of life. I escorted a very good friend to mental health when he discovered his wife was cheating on him and sat there with him for hours, she was his world and it took him down at the knees, thankfully he recovered. I've wept with parents as they processed the loss of a child. And sadly, I've sat with a son as he processed the death of his father. Mentoring is SO much more than career advice.
I'll get off my high-horse now because trust me, I have fucked up just as much as I've gotten right, I just have very strong feelings on the subject mainly because any success I had as an aviator was because of two people who mentored me and demanded I be a better person and aviator. Mentorship is not perfect and sadly mentors can fail...sometimes miserably. I counted Baba Rand as a mentor for many years and when it counted most, he wasn't there.
Maybe instead of asking if the system needs more mentorship, we should all just go out and do it.