[kelnos]

That makes no sense. How can you sue a non-practicing entity for infringing your patents? If they're non-practicing, they're not doing anything.

[dritanm]

Being a plaintiff doesn't always mean you're the one suing. It could be an appeals court case where the positions are reversed. Also, you could sue an NPE in order to invalidate their patent(s), or to seek an injunction.