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Rhovanion and Harondor are the only two factions that can choose between WotW and WoD culture buildings. It has no effect on troops/buildings, so it is more about ease of expansion. It is a very recent addition to these factions and could perhaps do with more of a think through - if we get time...
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Right, 'converting' doesn't really happen in the sense of a faction like Rhun or Dunland converting to the Cult (no new troops become available if over 50% of settlements have a different alignment). The real mechanical benefit is that e.g. Rhovanion could benefit from Trials of Blood upon conquering a settlement of Rhun, rather than having to build its own Ways of the West chain and having to deal with converting the populace. So ease of expansion, as MoN said. In its current state, the ability to choose would of course only be useful insofar as one of those factions decides to expand into territory belonging to a Ways of Darkness/Ways of the West faction.
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