It's EA, so Larian was probably satisfied with the verbal casting cancellation function, enough to put it into the game, while knowing that it doesn't actually affect sound yet.
I'm sure they would like for it to be fully functional, but if they do, it will take time to develop, as they have to find every scenario in the game that silence would affect. That's a fairly large endeavour if you think about it.
That said, this is definitely something I'd like to see fleshed out.
