Knowing that Davy Jones and Calypso would only be allowed to see each other for one day every 10 years, why did they even make that deal? Wouldn't they have been better off to just remain as they were? Was Calypso desperate for someone to take that task???
Why did they even make that deal? Well, "they" didn't—he did.
Traditionally, the task of ferrying souls would be considered a curse. (Look at, e.g., the 1921 Swedish silent movie "The Phantom Carriage" or Ray Bradbury's "The Scythe".) But in the Pirates movies, the agreement seems to have been that Davy would do the task for ten years for a chance to be with Calypso for 1 day.
And then she reneges.
Why does she renege? Because it's "her nature". Now, the thing about the early "Pirates" movies is that lying and being self-service is pretty much the way of things, so while she admits she missed their once-a-decade date because she's capricious, she also immediately follows that up by saying they could be together forever if she were free (and Jones had his heart).
Furthermore, when he confesses that he'll always love her, the smile on her face isn't exactly a warm one. Obviously open to interpretation, that smile looks more like one of having the upper hand.
So I don't think Calypso was desperate. I think Davy Jones was, and ended up regretting it.
Fandom link with synopsis that might be helpful: