Destiny patch 1.0.2 has now gone live which means that if you had any legendary engrams before the update they have now been reduced to rare status.
However, the update will ensure that any legendary engram presented to the Cryptarch on the Tower will yield a legendary or exotic weapon, armour, or item. Rare engrams will also always produce rare or above goodies, with an increased chance they'll unlock something legendary too.
Ascendant materials have been reclassified as legendary items in this patch, due to the fact that they're often needed to upgrade legendary items.
Daily Heroics and Weekly Heroics, among other events in the game, will also yield better results, with more rare and legendary engrams dropping during these events.
Last week Bungie nerfed the game's infamous Loot Cave but that location was quickly replaced with the Rocketyard Room. The Loot Cave lives on thanks to the Interactive Cave Shooting Simulator browser game although obviously, you won't pick up anything for use in Destiny.
Players in Destiny need rare, legendary, or exotic armour after level 20 in order to advance their light level and hit the cap of 30. Even with the patch, there's no guarantee that engrams will actually match your class.