Not knowing the scope of the application this may or may not be 'ammunition'. But it might help to inquire what they intend to do if down the road they want to interface with an app or object outside of the current platform. Say for instance a Java object or C++ objects that come from a Unix Application. With the SOAP solution you and potential clients are already set to rock. With their solution you are locked in...

And the Gate's tabernacle Choir Sings,
Forever, and Ever!,
hallelujah, hallelujah!
For you shall pay forever and ever.
For you shall pay forever and ever.
hallelujah, hallelujah!