peter:
That is indeed a detail. Between craftsmanship and the helo upgrade, it takes about the same amount of time to research helos.
Likewise, considering you only need a few 'phracts to take out a whole lot of cats and helos, the initial investment is a small matter. The gold cost is only slightly morethan researching scythes, and the food cost is... well, if you're not using eles vs helos food isn't such a big deal in iron imho, you can get a lot of it very quickly with either boats or farms.
Phalanxes get the job done just fine, the cent upgrade is overkill. AEs rock, but mostly on the offensive once you're winning anyway - for defense you're better off with phalanxes, EAs/compies and scythes/phracts.
Besides, we're getting lost here. I don't think cataphracts are underused - it should be experts using them instead of newbies, but they aren't underused.
One unit which I think is severely underused is EAs. They are imho the compies of the iron age, when massed they do a great job of taking out HHAs and even helos with minimal losses, as well as infantry and possibly even scythes. Their only weaknesses seem to be vs cats, AEs and priests, two of which can be easily dispatched by scythes. The selection of civs which has them is quite nice as well - persia, phoenie, hittite, egypt and carth, all great civs in their own way.