I had dinner with some of my family tonight. My mom, my dad, my oldest sister and her husband were there. That made me and my nevermo girlfriend the only non-Mormons in the room.
My mother mentioned that she'd recently read a quote from Walt Disney that went along the lines of "neglecting your family in favor of your career is bad." (According to BrainyQuote.com, it's "A man should never neglect his family for business.") And my sister said it reminded her of a quote from one of the Presidents of the Church about how "no success can compensate for failure in the home."
They had a brief discussion about which prophet said that, and, as I listened silently, they arrived at the correct consensus of David O. McKay. None of them had any clue that McKay's famous quote was lifted from Benjamin Disraeli. I didn't bother mentioning it because I have no proof and it wouldn't really change their minds about anything anyway. Also I'd prefer to avoid straining the relationships even more.
But to me, it's pretty clear: a prophet of God has no need for plagiarism. If he speaks the word of God, he doesn't need to speak the words of another man. And if he plagiarized "as a man," a phrase often used to dismiss poor actions taken by church authorities, then I find it highly suspect that God would allow such a dishonest man to be his mouthpiece to the masses.
This is far from the only example of a Mormon authority lifting his teachings from another source. And that does not seem right to me.