If you want to be technical, there ARE specs in the game. They are just designed to not look like it. ACN can "spec" to be a healer or continue being DPS by choosing SCH or SMN. But instead of a tree with choices, the game is designed to have it spelled out more clearly and have them defined in roles. It's techincally exactly what you want, and in the future only classes will have "specs" for them aside from the one they have now.

You don't have to like it, but that's just how it is. That's how the game is designed. And honestly, it's not much different, just given a clear label.

For example, let's say in some other MMO, your knight can spec as a tank, the normal tree, but with certain skills and gear, he can forgo his tankiness for straight up DPS. That's what you want as customization. But everyone that decides to make a Tank Knight will have the same gear and builds, as it fits that "spec" better. And everyone that goes DPS will have the same skills and gear, because it fits the DPS spec better. So what FFXIV did, instead of leavign that ambigous, made them into CLEARLY defined jobs. They each have different skills and gear, but come from the same class.

It's actually exactly the same, except your spec is given a clear name.