Ascend Charlie - I totally see your point ... but....
From the POV of those paying big money for their business class seats - they cherish the fact it is half empty. They do not want anyone legging it in from the cheap seats just because it's not full in business. If an airline had a rep for doing this they'd soon start using another airline.
However the cabin/checkin crew can and should use their discretion but sparingly so as not to piss off the money.
If you don't spend the money you don't have any moral justification for being aggrieved at not getting upgraded.