There's a difference between
a) Movement has forced me to break my standard rotation and use suboptimal movement-allowable filler
b) Movement has forced me to use an ability that is actively having negative repercussions after said movement ends, which is exactly what downscaling dots or draining shards does
Allowing haunt to be cast on the move fixes if we're forced to make movement for the 1 second or so cast haunt has, letting us avoid interupting it. It doesn't fix the core problem that we have nothing to do WHILE moving, like, lengthy or regular movement.
Wouldn't be so bad if our dots were actually potent without relying on our filler spell, but I doubt we'll get decent dots with shadowbolt back any time soon.