This situation occurs with vendors in the Pale Heart and Neomuna. The commonality between these two destinations is that even after unlocking all weapon patterns, certain activities (such as Overthrows and Terminal Overload) are still required to obtain random rewards, during which destination Engrams are generated.
Once we have unlocked all the patterns for the destination weapons, there seems to be no worthwhile option to utilize these Engrams. It is hoped that more purchasing options can be provided for these vendors.
Pale Heart: Exotic class items (41 Engrams for one, similar to Xur) and Ergo Sum.
Neomuna: Focused Decoding options for Terminal Overload rewards—Volta Bracket, Circular Logic, and Synchronic Roulette.
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I personally use those engrams for fast glimmer and or cores when I'm broke after spending on iron banner weapons 25k glimmer a pop goes quick. Just this week I was upgrading and enhancing weapons for next seasons artifact and dismantled over 400 engrams across all vendors in game now I'm no longer broke and ready to jump into the season prepared with fully enhanced weapons at level 20 for heresy.