I don't often use it - though with the screw mount cameras, I do have to, every time I meter
and with preset lenses, right before I shoot
I would not disqualify a camera if it lacks a preview button.
Not only for all the reasons you gave in the OP and everyone else, there's the idea that what you see on the ground glass isn't necessarily what you get. Better to keep certain principles in mind: the in focus area to the front is 1/3, to the rear is 2/3... (now I may have that entirely wrong, but somehow that rule of thumb stays with me)...