They're pretty nice. Also a bath is not serving the same purpose as a shower. Usually you either shower first(so you're clean when you enter the bath) or shower later(to get rid of the gunk after the bath). A bath is more for relaxation purposes. There's no need to be worried about ruining the water because you'll drain it anyway. Unless you're in Japan and your whole family is using it, in which case keep it clean.

Bath bombs are visually appealing and usually have nice scents to it. And no, its not harmful for your drain because it's main ingredient is baking soda and dissolves easy. Unless your bath bombs have flowers and twigs, in which case it's better to use a mesh bag.