I think your interpretation of "sign a document so I can sue you later" is pretty inaccurate. Someone is disclosing an idea and wants to protect themselves in case you decide their idea is worth implementing. I don't think it's an unrealistic request. Furthermore, most NDAs I've seen don't really protect much of anything.

The downside of saying 'no' is that you will never (or rarely) have them as a potential client. Maybe you have clients throwing themselves at you, but in my neck of the woods, clients aren't exactly a dime a dozen. I'm not going to sign anything that I feel would put me or my business at unreasonable risk, but no NDA I've ever signed has been worded that unrealistically.

To make a blanket statement that I'll never sign your NDA and NDAs are a dumb waste of time sounds like a difficult person to deal with right off the bat.