Horizontal progression was one of the main reasons I quit Ragnarok Online. As other people have said, it necessitates having abysmally low drop rates on items to ensure that people don't get their gear too quickly or easily, the end result being that you can grind for a particular piece for months and never see it drop once.

And then once you finally do have it... okay, that's great, then what? You have nothing to aim for because you've already got that piece of BiS gear. It leads to either tremendous stagnation or massive frustration after a while depending on whether you do or don't get the drops you're after.