You should be able to use the intercom built into the radio, but this would require more wiring and a Tx relay (normal/emerg sw) if you used this method you would need to ensure that your headsets are matched. I think the main reason for fitting a intercom is the reduced wiring and therefore reduced costs if you are paying someone to fit it! also most intercoms are voice activated so there is a reduction of noise in the headsets when no one is speaking. In my current aircraft I have used the radio intercom and it works very well but for my Kitfox I will be fitting a voice activated intercom.
Dave