It's not that the rare tracks haven't been released, rather a)they've never been officially released, or b) these specific versions of the songs haven't been released. I'm sure most people that have bought a couple of Nirvana bootlegs will have heard almost all these songs. There are a couple that I've never even heard the titles of, but I'm guessing I'll recognize them once I hear them.
I used to have a huge Nirvana collection when I was younger. Unfortunately I sold off most of it on ebay to buy food in college
)