> Even if that is true (which I am doubtful of), what is the merit of OER at all?
The merit of OER is that is a measure of the cost of using owned housing as housing instead of renting it out.
> Why use it if its A) not really what people are paying
It is a measure of what people are paying, as an opportunity cost.
> B) relying on people not in the market to guess what the going rate is
Potentially a legitimate concern.
> C) consistently underestimates actual rent.
Its not trying to measure actual rent and is not consistently below actual rent (which, again, is tracked separately.)