1. They may have looked at $ spend in order to determine which people got the offer under the theory that low BIS spend isn't worth over-populating a tier.
2. The most likely value is in GM to Silver or Plat to 1K. In the former, you can get -UP fares on Y/B and the latter, Y/B/M. Those instant UG's are a huge value to many.
3. Whether an offer is "worth it" is purely individual. If it's what you drop at a bar on a Friday night, it's worth it. If it's the difference between making your mortgage and losing it to foreclosure, I'd say not.