6 x 6 has lots going for it, but the extra negative real estate compared to 6 x 4.5 is only of value in use, if you print square, which mostly I don't.
Mostly I print on to 8 x 10 or 11 x 14. If I print from a 6 x 6 negative, I "waste" almost 25% of it. Whereas from 6 x 4.5, I waste almost none of it.
And you know the saying: "Waste Not, Want Not".
There are lots of good arguments for 6 x 6, but the "larger than 6 x 4.5 negative" one is poor, at best. Because in most cases, I and many others don't get to use that extra negative for any good purpose.
My 6 x 4.5 negatives give me negatives which are essentially of the same quality as my 6 x 6 negatives, save for the relatively rare times when I print square on to a larger paper size.
It doesn't matter whether the 6 x 4.5 negatives are exposed with my 645 camera, or by using the 645 back for my RB67.
And the 6 x 4.5 slides I get from either one are really, really nice when projected. A lot better than a 6 x 7 slide would be, as I don't have a 6 x 7 projector.