Desmond and the Monk really had no choice but to accept Klaus's offer.
First, he would make them his weapon spirits, whether they liked it or not. The only upside is that if they choose to become his weapon spirits, the process will be less painful.
But regardless of what they want, he would make them his weapon spirits, and they have no say in that. They would have no choice but to accept.
The second is the prospect of becoming a Celestial one day. They just saw Seraya transform from a weapon to a human. Naturally, that means they can do that too.
So, while they would forever be weapon spirits, if they prove to be good boys, Klaus might allow them to take on their human forms from time to time.
Third, Klaus didn't fear they would betray him and even told them his true identity. To the monk, that was a leap of faith and trust that can go a long way.
