[george3383]

Having an open source client and server would help how? How would you know the server is running that software? Or that the binaries distributed via the App Store don't have some special spice?

And as far as I know the whatsapp protocol is simply tokenised XMPP.

[Karliss]

If you have proper end to end encryption protocol and client which correctly implements the protocol without additional backdoors then it doesn't matter what the server runs. If the client is open source you can verify and build it yourself or download the binary from source that you trust.