Well, it's still better to try than not to try
BTW this tag could be tried on National Weather System (or what) too that provides a free promotion for air units.
I would only use this on World Wonders. With National Wonders, everyone gets one, so the challenge is picking out which NW you're going to put together and you don't have room for all of them in one city. With World Wonders, the challenge is getting to the wonder before your rivals do, so the effect deserves to be a little more widespread.
