Yeah, having to fly NW metal only to redeem a voucher to Mexico or the Caribbean can be tough. From ATL, for example, most routes would have to go through DTW or MSP (not many are out of MEM), and NW hasn't been matching sales. US tends to be the fare leader, with CO and DL sometimes matching. Not much help for spending vouchers. CLT (a US hub) is a better situation on willingness to match but still not the best.

YMMV of course depending on origin, but I have had the best luck out of ATL to places like ANC (doing a CLT-ANC for $99 net after $300 reduced), the west coast, or possibly west coast-Hawaii.