Upgrade certificates are valid for 1 year of date of issue.

You´re right, being Ex. Plat by passing the 50K mile mark is probably not worth it for international flying. Most of the benefits are for Zone 1 flying (unlimited upgrades, etc). However if you decide to go for the Ex Plat, remember that when you are a Gold or Plat, you earn 75% and 100% more miles, respectively, on business class fares, and those miles count towards your elite qualification. So one trip while a Plat will earn you double miles towards Ex. Plat qualification.

I´m watching with anticipation to see what will be offered with the Taca merger. It might be good to be a Ex. Plat when they finalize their FF programs.