Some of the biggest problemS (plural) that I personally have found with the plagiarism issue are that:
a) I've gotten different descriptions of it, depending on who I talk with. It makes for loooong (and kind of frustrating) conversations, because I'm constantly asking, "Can you explain what you mean by that in more specific detail? Can you give me an illustration/application?" I'm genuinely not trying to be annoying or dodge or bog down the issue - I just want to make sure both I and the person I'm talking with are on the same page.
b) There are some sermons that many, MANY pastors do use - at least the outlines of them. For example, there is a sermon called "A 4 Way Look At Communion"; the basic outline of which goes like this:
Look Back - At Christ and what He did for you
Look Around - Communion is a Family Meal
Look Inward - Self examination
Look Ahead - He's coming back
Now...Call me crazy if you want, but I personally believe it is possible that that sermon outline could well be over a thousand years old. I mean, it's SO basic that I can't see it not being literally ancient.
c) The Holy Spirit has gifted wonderful Bible expositors and teachers over the years (centuries). I believe it would be kind of foolish to not read and use their insights. And yes, there are times when they say things better than what I can - Although I do very, very. very much believe in filtering what someone else does to the point where the result is YOUR OWN as much as possible.
My apologies if this post throws things off-topic.