It's hoarding if you have an emotional tie to every inch of yarn and you can't let it go. If you use the yarn for projects and have a use for them, then your yarn is just part of the supplies for your craft.
Just like the screws, nails, duct tape, hammers, wrenches, etc., that litter every man's tool box.
I have tried to explain just that to my hubby! I am trying to educate him, he is a builder, so uses different materials for different projects. Its just the same for us.
Now how do I sneak in 20 balls of Aran my daughter bought me with her yarn lottery winnings?