To make each set of gear feel like it belongs to that class. There are some pieces that aesthetically look good on a Monk or Samurai, but wouldn't look good on the opposite. Same with Aiming gear, there's some that would probably look good on Bard, but doesn't fit the style of a Machinist.
I suppose, but at the same time, with TP being removed in 5.0, we might see Skill Speed being used more often, maybe. It could be useful on classes that generate resources on weaponskills (Paladin, Warrior, Dark Knight, Samurai) or auto-attacks (Ninja).