There's absolutely nothing wrong with a bit of sensible plagiarism. Plagiarism is a very cultural thing for us Westerners [especially]. In some countries [Middle East] you will find that you do not get top marks unless you copy everything your professor told you, letter to letter.
If someone makes a scientific discovery and states: "I have proven that the sky is blue". Now I am writing a paper and I want to use his research as a reference and in order to not plagiarise I have to paraphrase (I could quote but I won't in this example). So how do I do that for such a simple and short statement? "It has been proven that the heavens are azure." This decreases readability, often adds to length and decreases efficiency.
Conclusion: Plagiarism is a part of our [bullshit] Western culture and it is counter productive; referencing alone should be enough.