I think it's worth remembering that these are not mutually exclusive options. You could start at early and have a traditional grand campaign in MTW. The difference was that you could also start in high or late.

Having three scenarios gave greater strategic depth as over the course of a single game, as different factions would be dominant depending on the relative strength or weakness of their units at the relevant level of the tech tree.

For example, in an early era game the Almohads would often steamroller the Iberian peninsular (assuming an AI Spain), and as France you would find yourself refighting the battle of Roncesvalles time and again. In a late era game, expect to the Spanish roaming happily through Cairo. Early-era Germany was a deathtrap. Late-era Germany had hordes of Gothic Knights beating up the Belgians.

That's even without mentioning the Glorious Achievements mode - although I personally think that GAs should give some tangible reward in-game, which is why I like the new treatment of Crusades.