Motivating normalization of splice
Unnormalized monoidal spliced arrows introduce a lot of unnecessary bureaucracy around the units: we need to track whether a hole is tensored or composed with morphisms. As a result, single-hole expressions may have multiple types, which was not true for spliced arrows.
The situation is even worse in symmetric monoidal categories: we do not want to track whether a wire crosses to the left or to the right of a hole.