Horizontal progression has limits, and once those limits are exceeded you will see playerbases just falling off, because repeating the same content is lame. So what if they add in 3 new dungeons in horizontal progression and only 1 of the pieces of weapons/armor are of use because it doesnt benefit you at all. What good did this horizontal progression do?

When you are stuck at a certain rank, you are limited with development and ideas on what you can do with those ranks. Forget adding in new skills that would destroy previous content, you will hear bitching regardless. Vertical progression gives developers and players always something new to think about and do.

With vertical progression, it creates a need to grind on "NEW" mobs, NEW areas, NEW Content, NEW Dungeons, NEW items, NEW everything for those new ranks...

With horizontal progression, you have no need to grind or rank on new mobs, no need to goto new areas, new dungeons may be pointless and new items may/may not be of any use to you... Horizontal progression are for LAZY developers, LAZY players, and LAZY altogether.

Vertical progression is the only thing that keeps NEW alive... OLD will become OLD anyways, and eventually no one wants to do it. Might as well keep NEW alive, and NEW stuff to do...