How though? It's filler. The spell you use the most. Even more so with Shadowbolt, since it's shorter cast time and you end up casting more of it. Dots may be the bread and butter of the spec mechanically and thematically, but when it comes to how the spec plays and how it feels, the filler may have a much higher impact. And since Shadowbolt is the longest non-channel filler, it may feel like bad. It also has no movement benefits of a channel and, given the dot nature of the spec, people may find it to be thematically off.