To be honest I don't know how gear set bonuses will work. because I kind of agree with you that the set bonuses will have to be substantial in order to warrant collecting all the pieces..
Here's the problem with set pieces, we don't have skill trees. The reason WoW has them is that each class has 3 specializations; FFXIV eschews that design in favor of multiple classes serving the same roles. They would have to make one sets role based, and then offer bonuses based on your class, and then get people to have all classes in that role leveled. Even then, you run into a problem, because that gets the numbers crunching and the BiS going. I want more gear variety and armor like FFXI had, but I also understand that adds complications to the game and the devs have other things they want to focus on.