While I would never embrace plagiarism (even in regard to repeating someone else's sermon), I think that there is a problem in Christianity in which so many people repeat the language used by others.
If you watch Christian television or listen to Christian radio, you will often hear the same rhetoric being used by various personalities. It is like they are using the language that is popular -- not with the world but among Christians. My wife calls this "Christianese."
I mentioned this to a friend once who rebutted that sermons are often "inspired" and, thus, "spiritual" rhetoric would naturally become "common" by which it is inspired by God to multiple recipients. I, in turn, rebutted that the same could be said of unspiritual or sinful rhetoric that is inspired by the whims of this dark world (or even spiritual wickedness in high places).
I knew one charismatic preacher who would talk about "swimming in the river" (referring to "God's blessings"). Within weeks, I knew people who repeated this in their daily conversations. This and other rhetoric became common language used by quite a few people.
Like doctrinal adherence required for membership and activity in many churches with sectarian doctrines, people have a tendency to resemble the things that they yield themselves to. This is why it is vital that we yield ourselves to Christ -- first and foremost. Our language will be biblical and our love for Christ will be apparent.
Now, I have heard of preachers who acknowledge preaching messages written or recited by others. They don't credit the source either (even claiming that God was the inspiration of the original message). I do think that there is some danger in this practice. A quick citation (even rhetorically) would be more forthcoming and honest.
I also think that this raises an even bigger issue. What happens if you are a pastor or teacher "scheduled" to speak and feel no unction or inspiration about what to speak about? Should such an individual simply sit down? Should they ask if anyone else among the teachers of the church have something to share? Perhaps they should simply read the Word of God during such periods of time. In fact, it could be very beneficial if pastors without a message would simply read (publicly) the Word.
I will say this: I have often "received" more (in terms of knowledge, passion, and hunger for Christ) after listening to 30-50 year old messages here on SermonIndex from men like Leonard Ravenhill than I have heard coming from pulpits in many congregations in America. In fact, this might be a great option too. If a speaker has nothing to say during a scheduled meeting, why not share the words of some other minister who has gone before us?
I don't think that it is being too "legalistic" to want a speaker to speak under the pretense that the words that he is sharing is his own "revelation," "inspiration" or "understanding" from God. At the same time, I don't think that anyone should spend too much time focusing on such things. Like what I suspect is true with most people, I often read the Word and pray during any message that I hear and try to sift through the words spoken and find some wisdom from the Lord.