Just a follow-up on this thread as it's been a while.
Whilst my views are same as most and certinally the having to explain the use of the voucher to the check-in person is frustrating I have been able to use some lately.
Sofitel ZQN, upgraded to club room from entry level (so more than one cat upgrade)
Novotel AKL Airport, upgraded to a suite from an executive room (so one cat upgrade. Not worth it to use one from Standard to Exec room as they are almost identical but a suite is a decent upgrade)
Sofitel AKL, Once cat upgrade (from standard room)
Both myself and a colleague looking to use up some old vouchers have found that a polite email to the hotel before arrival (with a copy of the voucher) and a note saying that we know they are subject to availability and can a not of our desire to redeem them be added to the file is usually met with a positive response (and even once an arranged in advance upgrade).
SSNZ