They're bad because when left uncontrolled they multiple like crazy. Like this thistle shown below. It's popping up everywhere and it's real nasty to pull out because of prickly leaves. It can enforce its punishment even through garden gloves.
They're ugly because they do tend to make the area look somewhat unsightly. I struggle with my urge to obliterate them, reminding myself that they do provide a service in attracting more of the butterflies and bees that I want in my garden.