Generally vanity items/items that don't effect max level players or the content, is for low levels. The only time I've seen items that were from events for high level, or close to it, has been items that max players can actually use (even if they don't have the best gear/raid gear).

Things probably would've gone better if the event items were completed at lvl 15, but the quests were more numerous to fill up a few days of questing for them. So, let's say the final event reward was Lightning's weapon (that gunsword thing). You'd get the normal white version (thats in the game now) and at 50, you could get a HM Primal version of it. Or something to that effect. Low levels get to see the end of the questline and a useful low level weapon, high levels get a weapon that's useful to them upon hitting level 50. Both now have an item that can be used for when vanity comes out. Everyone wins.