There were lots of things I loved about xi. Fighting tooth and nail to justify my reason to exist in group play was not one of them (I played thief and dragoon which were generally in the bad tier for the majority of the games lifespan). I don't wish that kind of discrimination based purely on the class you play on anyone. You guys remember the no melee allowed, bard only dps and pld only tank groups after 2.0? That, but far worse ad extending for years, not months and ultimately still never fixed.
The reason I don't see the value in a spec system in 14 is because of the armory system. In a 1 char 1 job game like qow, there is a serious advantage to being able to vary the playstyle of a single class. You're stuck with it. If I could only be a war on this char, and to play a dps I had to make an alt, do MSQ again, and all that crap, I'd be much more inclined to have a dps spec on war instead. But as that isn't the case, I feel it's really just a duplication of the armory system we already have. Sure you have to level it up, but that's nothing compared to a 1 class 1 char design. That's why it works in wow, but would be just unnecessary system duplication here. It kinda undermines the whole purpose of the armory system. It's just aesthetic at that point. "I wanna melee like a drg but look like a drk doing it with drk dps spec!" Or, you could just plau one of the 3 melees we already have when you get a hankering for some hurtin.
Trees and customization stuff generally has wide ranging balance issues and that's no fun for anyone but the flavor of th month job/combination. Specs aren't nearly as hard to balance (they are static like a new job not hundreds of olayer chosen combinations to worry about) and have a valuable place in 1 char 1 class games so you can change it up occasionally without starting from scratch. But the armory system already fills that purpose here. Why have 2 systems to do the same thing? Sounds like a lotta work for for nothing.
Specs: change more easily. No real 'cost'. It's convenient
Armory: more work, but also FAR more options and varying play available. Sure you can't play a melee dps drk flipping a switch. But for some effort you can play a melee, or a bard, or even a healer! Or nuke stuff woth black.magic. it's far more flexible than the 2 or 3 options specs afford at the cost of more effort. That's fair tradeoff to me. I never have to redo all starting over char junk to do something different.
Tldr: specs serve the same function as armory system. It's redundant.