The Whip controls a vote of this import. Not gonna vote if the Whip doesn't confirm the votes are there.
Normally, the Speaker will listen to the Whip's count. But the Speaker can proceed with a vote for strategic reasons nevertheless if he so chooses. And I suspect a failed vote is more likely to generate movement than not having a vote at all.
One other potential plus -- letting all those guys get on record with their votes may make it easier for them to compromise later.