FFXI worked for horizontal progression for one main reason - the ability to macro change gear in battle on the fly. This allowed people to min/max gear to fit the specific action (although the blinking used to drive healers nuts). Swapped gear would often contain buffs specific to the actions performed - more so than changing primary or secondary stats. So, you would swap the gear in to do a buff (put up Sneak Attack for example) and swap it back for max damage when you executed your next attack. If that attack was a weaponskill, you would macro in the gear that affected that weaponskill, then swap back. You could easily be carrying - and using - 5 or 6 pieces of gear for every gear slot (the only exception being your main weapon as changing that would reset your TP).

Frankly, I don't miss this and wouldn't want to see it added to FFXIV.