There is a such thing as legitimate private add-ons; developers who make their own stuff and keep it to themselves for numerous reasons, which I've done: very niche things specific to a build (an event that spawns bots at minimum size that wander within a few bricks of the spawn), things that aren't user friendly at all, things that most people would find annoying (a chatbox that learns from chat, which although it makes some funny randomly generated sentences, it's still a chatbot which many people hate), things made out of boredom/practice with little practical application (a REST API framework for Blockland? Neat, but what would a blockland server need a REST API for?)
But most "private" add-ons that people discuss are things that were once publically released, and then either failbinned, or the link/hosting service hosted on died.
And then people refuse to give out the link because they
think it's something to gloat over
This:
http://forum.blockland.us/index.php?topic=159806.0
is the best example in this thread