So a lot of us been there. Got a legendary engram by killing something and got ourselves excited thinking "Will it be good?". But you go to the Cryptarch and... Green item. I know how it feels. You can cry now.
-
In my opinion engrams should only ever give either the quality that is shown through its color or something higher. For instance, blues should only ever give blues or above, etc. The balance to this would be to make the blue/purple engram drops more rare. Personally, I'd rather fight more mobs for those blue/purple engrams and be guaranteed something of that quality than finally find one just to be let down.