We have a 24V Itasca and use both queen beds. The DW has a bad back and needs the full spread-out room of the rear corner bed, so I use the cabover bed. Both are real comfortable with great mattresses on them as installed by Winnebago.
However ... our 24V is a basement design Class C, so the floor is closer to the cabover bed than with the non-basement design ... so my climb up into the cabover bed is not very high and very doable. Our cabover bed's height to the ceiling is also pretty good, as I can sit up in it with my head not quite touching the ceiling. Of course, the dinette can always be converted into a double bed for emergency sleeping by two more (grandkids?).
It seems to me that the cabover bed design in a Class C with both queens, at the very least, certainly offers more flexibility in use of one's Class C. If you don't want to use the cabover bed for sleeping, then it's a huge storage area. I you do want to use it for sleeping, then you have two queens available if and as needed.
What Class C manufacturers could do but don't well is, in Class C models with cabover beds, still make the front of the motorhome more streamlined and molded from a single piece of fiberglass ... just like with "Class B+" rigs. The Phoenix Cruiser is one example: Why not have their small, single fiberglass piece, and streamlined cabover area still be a single bed - as an option? That way one would still get some gas mileage benefit, but have the option of two permanent sleeping locations for flexibility - the corner bed in the rear being kindof a queen and the cabover bed being kindof a single.
IMHO, the name of the game in any well designed RV is flexibility - with as much as possible double or multiple use of it's layout configurations and always a Plan B on how things can get used by the owner.