Probably the most important reason to introduce the odd horizontal progression patch or expansion is to help prevent straining the level system. Eventually, leveling has to be made faster to get people to max level in a reasonable time, and that messes up the pacing of the game. There are some other side effects of raising the level cap, like the inclusion of new, albeit disconnected, zones to the game. It's alright to add a few such zones, but after a while it can start to increasingly isolate players from each other. So any suggestion to add horizontal progression should probably be accompanied with a request to spend time expanding current zones instead of only adding new ones, even if that means having to break it up into several smaller zones that share the same chat.

Both WoW and EQ2 have level bumps that go straight to end game, EQ2's being free of charge and WoW's being a paid service. These were added because players simply didn't want to go through the leveling experience, which became problematic after many years of raising the level cap. So much of WoW is like an abandoned ghost town, it doesn't make sense to follow in its footsteps.