Because if Talent wins there will be one more defection. They need Barkley to stop that.
He can't. If the Republican lead in January is going to be 1, then Daschle will offer Chaffee the moon. If it's going to be larger, they won't bother, or he won't take it. Barkley is irrelevant to those calculations, because he won't be there in January. He's a place-holder for 4-6 weeks, and his positions are utterly and completely irrelevant.