Vertical progression should match with the maximum level a player can achieve, though. The problem with the gear score boosting method of progression is that eventually it leads to huge peaks that suddenly have to be addressed when the next expansion comes out, to the point that eventually all the gear has to be recalculated down the line. Horizontal progression and challenges lets people develop multiple sets of gear as well as gear up alternate classes/jobs without suffering from content burn out and helps address the score spikes that happen at end game.