I believe the main story thread to D2 was about making alliances to stand against the darkness or more likely its followers.
We allied with calus and his cabal, we met and allied with mithrax who created house light. We strengthened awoken-guardian relationships. We allied with Rasputin. If Asher's joined the vex collective and serves as an ambassador we could ally with that sect.
Also on a side note, asher joining could have negative effects for guardians. As vex could not predict the light or simulate it, but by asher merging with them, they may be able to simulate it now that they have access to his memories and possibly the light.
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% Lets not forget about Osiris, Saint, Riven, Eris's return, etc. T
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It might be a risk but light wielding Vex is just so cool to think about.
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If they do gain that ability then its possible we could see vex capable of wielding both the light and dark or even see a civil war between dark wielding vex and light wielding vex
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That would be a really cool cutscene if they make it into a battle.