The only purpose that the higher level equipment would serve is like you said, continued play of the hard raid along with PVP activities that had level advantages enabled like Iron Banner and Trials of Osiris. Other then that there is no real reason go for those things unless you just want them to say you have them. That's why things like the elemental primaries were good and served as good rewards from activities like Raids and Trials of Osiris cause they were that icing on the cake that typically made the effort worthwhile.
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I am not convinced there is a significant difference in pvp performance between 290 and 320.
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Maybe, maybe not. I hardly touch PVP so I don't know if Light Level has that much of an impact in the Iron Banner or Trials of Osiris as a whole. It was just my assumption since level advantages are enabled that having high light level equipment would have an impact, however if it doesn't then really it just lessens the importance of the hard mode raid items even more.