I think Whatsapp gained it's popularity because you don't need a new id to add your contacts (the phone number is used as id) so if people are to migrate they would choose some similar app.
I agree with you, I care about privacy, buy nowadays when there are companies that have a lot more sensitive information about you (like fb, g+, etc.), the phone number seems not a big deal.
Anyway my motivation to do this port is just the personal challenge it represent and making an app useful to someone and my beloved N900.
By the way, If you or anyone have an idea of making an app fully open that could replace Whatsapp or other similar apps I'm glad to collaborate