This is an interesting question. I sometimes wonder that myself. Does 6x4.5 offer enough of an advantage?
Among folding cameras, you can get a very compact tool that yields a larger negative.
However, on the other end, I do wonder if a much larger camera, such as the Pentax 645N, is a worthy tradeoff. It's a nice camera, but it's a beast with significant weight. Would I be better off with a high-end 35mm rangefinder or SLR?
In the end, I think it's a decision the photographer must make.
I will say that the larger negative gives better tonality, particularly with black and white film.
In many ways, photography is all about compromise, and that certainly is true in this debate.