I think the OP's concern is that they are forcing themselves into a pattern for aquiring abilities and traits on a class, not necessarily the abundance or lack of abilities on the classes. My guess is someone just came up with this idea to give an ability every other level and it worked well enough for what they had in mind so they went with it. Not much reason to that but it is all I can think of. Maybe they liked that it would be "fair" to all of the classes, that they all get the same amount of abilities and thought they will be more equal and balanced.

Who knows if the pattern will stay post-50 or if abilities will be added/altered before then. If it works out poorly later it is likely going to change. You know their motto: "Content subject to change"