In theory what a NPE does is actually quite admirable. You're an inventor and you invented something and you have a patent and big companies rip you off. They know you can't afford to fight them so they just do what they want. So you sell your patent to someone like Intellectual Ventures who goes after the big companies for you. Now no one can make your widget bolt without paying you, as it should be.
And look what's happened - even giant companies are scared shitless to defend against patent lawsuits. In that respect, the idea worked.
In practice though what happens is minute, even trivial things get patented and NPE's go looking for people to sue, using a byzantine series of shell companies and borderline gaming of the legal system. Whereas the inventor of the widget bolt has to make the exact specifications of how his bolt works open to the public (who could also just figure it out by looking at it) software companies don't have to make the source code of their patented inventions available to anyone.
NPE's to me are like the NRA or PETA - organizations/concepts which started out with noble intentions (responsible gun ownership, don't torture animals) and just strayed way off the mark.