Specialization happens no matter what the armory system.

Ask the people in the Linkshell "who The Tank is?" and they'll point to a half dozen people if they're lucky. "Who's the WHM?" Maybe 3 or 4.

While it's possible to completely master every job/class (and by "class" I mean THM) no one really does it. The armory system allows you to try out every class/job so that you can choose which one you really want to play. And that's what it's good for.