Gambling early infrastructure on a nearly-as-powerful unit that takes 1/3 the research as Pikemen and has a better upgrade path is a fine trade. Especially since then you're along the path for other era-appropriate super-units. The problem is that along with making that tech/infrastructure sacrifice, you're also gambling that you have iron, which you may not and won't know until you've completed the gambit. One or the other of those risks would be fine. So you can put me down squarely in the "reveal iron at mining/BW and it'll be fine" camp.