As a NW Gold/Plat/2%, you participate in reciprocal upgrades on AS. AS Elites get to fly NW metal the equivalent of a NW silver, and the return is roughly the same; you fly AS metal as an AS MVP. The system used to require paper certificates for upgrade on either carrier, with a limited number distributed annually. However in 2008 this has been updated, so if you were to make a booking as a mid to high tier elite, you should be able to request upgrades and access priority seating on AS metal. The only real use for the certificates currently (and this may be obsolete now too) is if you were to book a Full Fare ticket (Y class) and there is U class available... then you can call AS and upgrade it. (U is a subset of F that is highly variable and restricted, especially on longer/preferred flights)

Not 100% on how this applies to NW Silvers

Also, the online U avail can lag quite a bit, so call AS before booking to verify U is available.