What's the deal with NDA's?

I've had three entrepreneurs ask me to sign NDA's recently. As an entrepreneur, everyone knows that NDAs really aren't worth the paper they're printed on.  I occasionally used them with big companies, as a means of communicating to the big company that I cared about the value of our intellectual property. But, they generally just slowed things down and became an item that future acquirers asked about why I was inconsistent in the use of them. So, I stopped using them. Now, as an investor, entrepreneurs should know that most knowledgeable investors won't sign NDAs. As an investor, I see lots of deals -- some of which are similar to others and I don't want to have an entrepreneur :

  1. Feel like they gave me an idea that is theirs and to have a piece of paper that leads to some legal letter. That's lots of unnecesary brain damage for me.
  2. Angst about their intellectual property so much that they won't talk about it without legal paperwork.
  3. Believe their intellectual property is so unique that they're afraid to talk about it. I think ideas are only good when feedback is provided and the whole NDA process seems to limit the entrepreneur's ability to get lots of feedback