While this is true when using nwa.com, I was once (Oct 2001) able to use a Northwest dollar voucher at the ticket office after I had made a reservation online using the Entertainment coupon and putting it on hold. I received the online booking bonus in that case.

I don't know if this still works or not.

Incidentally, I had reported this on FT, and the thread is one that is referenced by UpgradeMe.