I will defend the 6x4.5 format, but only for MY use. I really have trouble framing 6x6. I just tend to go to the rectangle. Plus, I got into the Bronica ETR system which is low cost and a bit more portable than a comprable 6x6 system. It fits my needs well.
That said, I see no reason why anybody would say 6x4.5 is better than 6x6 outside of some personal preference.