War elephants are more useful for their time, I think, because of 2 things:
1) Resources. Generally, Ivory is clumped, and you can have 2-4 nodes in one place. Iron mines can be pillaged. So can Ivory camps, but they are on flatlands, which menas invaders are prey for those very same war elephants. And there's more of them. If Iron is on a hill, the AI can simply move a few units up there and destroy the mine. Sure, you could use 8 or 9 units to defend that hill, or make a fort up there, but let's face it: even with that, Iron is vulnerable. You might get 2 resources in an average empire (and by that I mean 5-8 cities by Construction, with all BFC's and a few borders past that). Generally, you get 1. Sometimes, you get none. But Ivory isn't as rare as most people think... in my games, 6-7 civs have ivory in their BFC's, and about 6 more have it within grabbing distance (I play huge temperate fractal 18 civs maps).
2) I do not play as the Romans very often. Mostly as Cyrus for war games. If I use the Romans, it's AC, and then I shoot for building wonders until the later game, by which Praets are obsolete.
Elephants do get special mounted promotions though... the flanking promotion is often very useful against stacks outside of cities. Sentry, not so much, but I still occasionally use it. Plus, you can level elephants up faster because there are less units that can successfully counter them, and then you can get Blitz, and then upgrade them to Cavalry. I admit the last one is rarely done, but still. I usually have one or two elephants with a lot of XP hanging around by MilTrad.
Another slight consideration is Barb busting. Barbarian axes and archers/ maces and longbows spawn with depressing regularity, but barb spearmen rarely appear, and I have never seen a barbarian pike. One war elephant can wander around and take out barbarians even if they are on hills (although I generally do not risk them being on forested hills).