Gating content by having a single dungeon drop the marks required to advance to the next tier is horrible design, period -- especially when said dungeon requires a wait in excess of 1-3 hours to even enter it.

A solution would be to implement a random dungeon queue similar to Rift's. Upon queueing, players would have a chance at every dungeon in the game, and would receive tomestones upon completion. If the dungeon is sub-50, they would be synced to the proper level. Not only would this help folks get the gear they need at 50, it would help lower level folks progress and level up.