Are NDAs useful?

Partly psychological ... if they see a document marked confidential, they should mentally zip their mouths, partly to establish the right relationship. Some subcontractors attempt to bypass you to steal the customer. With NDA and trade secrets, the presumption in courts is that there is economic benefit so getting an injunction is a little easier than patent litigation where you have to prove infringement. Relevance is as much as parties put into their word ... it basically comes down to the minimum level of trust you expect in a professional setting ... and btw, word gets out in a small business circle if blatent in breaking law