If he cannot hit the threes then we will be seeing a lot of Casspi at the PF spot. And against a team like the Pacers that would be a defensive disaster. Casspi would not even be a bump in West's road to the basket.
DMo already has a reputation of a 3 point shooting big (at least commentators always mention that, so other teams must think so too), then why ruin it by shooting a 3 ever again?