I want the right to do those things, but I don't actually want to do them.
Now of course, they always say, "Yeah, maybe not you, but somebody." To which I say, "That's why you took so long to check my references before hiring me. I'm no longer somebody. And if it mattered that much, you should have put it in the employment agreement so that I could have declined the job before taking it."
The only people who should feel morally compelled to sign no-disparagement clauses before accepting severance are people who are being fired for leaking confidential information on the job.