Esse problema de pegar engrama de uma classe e ele virar algo inferior vai ser corrigido:
"After the patch, engrams will always decode into gear that is at or above the rarity level of the engram — so, no more decoding a purple engram only to receive a Rare weapon. In addition, there will be a higher probability for Rare engrams to produce Legendary items upon decryption."
http://www.polygon.com/2014/9/26/6852027/destiny-patch-loot-system-engrams-legendary-rare
Mas tem um detalhe, não adianta segurar os engramas e esperar o patch, pq todos que vc segurar vão ser rebaixados:
"All of this applies only to engrams picked up after the patch. In fact, all Legendary engrams now sitting in players' inventories will be downgraded to Rare quality when the patch goes live"