Paul - great job with Knapweed video recently posted, I like how keeping things moist will discourage Knapweed. While it is useful for bees to make delicious honey, it might be able to make fabric at one point, although it would be a rough one, similar to Nettle. Still, we need rope and burlap and things like that, may as well use it!
Medicinally, it is great for topical skin issues (some herbalists use this in salves and swear by it) and is even being investigated with combatting cancer! While it is a bugger since it is so invasive due to the plant's ability to render the soil unaccessible nutritionally for other plants surrounding it, I think an ability to combat cancer is worthy of investigating further! I have yet to get the book on Inner Traditions called Invasive Plant Medicine but hope to SOON!