By offering Werth arbitration, the Phillies put themselves in a win-win scenario. If he had accepted, he would have been a Phillie for one more year, either at his asking price or the Phillies'. No middle ground. That's a risk the Phillies certainly were certainly willing to take, since the contract would have been for one year only. However, they completely expected Werth to decline the offer. By doing so, the Phillies will now get two (first or second round) draft picks from the team that signs him.
Since Werth had until yesterday to accept the arbitration offer, this likely explains where that rumored "Tuesday deadline" for Werth to re-sign with the Phillies came from. He can still sign with them (he won't), he would just have to do it as a free agent.