I'd just like to add that I think a lot of their decision making came from being limited from the current UI. Up until 1.20, they've used ability cost because there was really no way to fill all 30 ability slots, which is all that UI could handle. Now we're getting enough abilities between classes, jobs and cross-class, as well as traits (something else limited by cost before) to fill up the entire UI.

I would speculate since they're redoing so much of the game that they're probably going to be redoing the core UI for abilities as well, since with a level cap raise (probably not happening until 2.0) they need to accommodate for more abilities and the current UI just won't allow for it.

So the 15/5/10 and 11 traits is just what we have to work with for now. There's a very good chance things will change quite a bit in 2.0.