1. Horses cause it. Ragwort colonises horse pasture because everything else has been nibbled flat and the horses won't touch it because it tastes vile.
2. It's almost impossible to get rid of. Our tactic of pulling it up by the roots is just about the worst thing you can do because you're bound to leave fragments of root in the ground and every fragment can generate a new plant. Cutting it down is little better; it just regrows.
3. Herbicides will kill it off. Trouble is they kill off everything else as well. Herbicides (like pesticides) are vile.
4. It can harm livestock. But the dangers are unquantified and overstated. No-one knows how many horses it kills in a year but it's probably not very many.
5. Horses won't eat it fresh (see above)- and when they eat it dry it's usually by mistake because it's been bundled up in their hay.
6. It has its ecological niche. There are species of caterpillar that won't eat anything else. Get rid of the ragwort and you get rid of the caterpillars and that puts pressure on the bats and birds that eat the catterpillars after they've turned into moths. Nature knows what it's doing. Best not to intervene.