I think you are right.
I have both 4x5 and 8x10 and I use them diferently: 4x5 for my work as an architectural photographer and 8x10 for my free work.
Sometimes I use the 8x10 to get 4x10, esp with housing estates.
Somehow I never saw the need for 5x7, it is too much inbetween these formats.
Contactprints I make seldom these day's of hybrid though.
Having said that, my customers in Brazil demand D only, I am upfront with the latest, if I would show up with a 4x5 I would get
teh name of "The old guy" and loose the business that I have right now....
Sometimes the camera is choosen for you by your customers, what you do in your own time is something else.