They're a joke, just as the legendary engrams only giving you blue ones. This game is so unrewarding to the point that I don't even get excited about these events or legendaries.
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If you got a legendary for every purple engram everyone would be level 28 by now. This is an MMO..
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I understand the purpose for them not being legendary every single time, but I've gotten more blue stuff from a purple than actual purple stuff. I wouldn't even mind if you only got strange coins or motes, but the blue stuff is just totally useless. Same with events, I did about 4 today and 5 yesterday, didn't get a single thing.