I have personally met, and briefly trained with, someone who hasn't tested since reaching Nidan...some 15 years ago.
Most would agree his Aikido is at the very least comporable to the Sensei's. Arguably (sp?) THE most powerful Aikidoka I have ever learnt from.
Once again brings up just how grading and testing can be important or trivial, or both at once.
I have no idea if that person is asked/approached on a regular basis to test or not. I'm sure he has his reasons.