Never trust your ghostwriter.
If you do use a ghostwriter, at least take the speech they write for you and drop it in Google or do what I do and subscribe to Grammarly.
Grammarly is a great tool; it costs $139.95 a year, and it's totally worth it.
Now back to Melania's speech last night. She trusted her speech writers and I don't know what kind of game they were playing, but they lifted words from Michelle Obama's speech from the 2008 DNC Convention. At the end of the day, she still has to take some responsibility for this even if she didn't write the speech.
The bottom line is, never let someone be YOUR voice. Even if you use a ghostwriter, make sure YOU read every word. They are YOUR words at the end of the day, and you have to own your words.