I suppose things like these have been discussed within the manufacturers' development units. My guess is that they hesitate to introduce functions that aren't really necessary and that will inevitably cause discussions and complaints. "The in-camera DOF meter said a DOF of 59 cm. Then why isn't the entire group sharp on my photo when the distance between front and back row was only 41 cm? I've studied the photo in 100% magnification and can clearly see that the back row is less sharp - this camera sucks!" But for those who can handle such a function and take estimations for what they are, I agree it would be helpful.

You could easily think of a customizable function where you could set the camera to show different parameters in the viewfinder, e.g. the set focusing distance, near/far DOF limits, DOF, hyperfocal distance etc. In this customization they could also allow the user to choose between the default 25 cm measures or set their own parameters for that too.

I'm not sure about the focusing distance measurement, maybe it's too rough. If so, you could imagine the complaints: "My camera focuses at 1.2 m when I carefully measured the actual distance to be 1.284 m. Please make a warranty repair of my eqipment." But even if the distance estimation isn't usable, it could still be helpful if the camera could assist in estimation of hyperfocal distance. (Still this would require that the users can accept the fact that the presented numbers are only estimations for typical use of the image.)