I mean, neither public map rooms nor arenas are strictly necessary. My personal view is that this isn’t about necessity, but rather about whether it has a non-decorative purpose without breaking any other rules.
No matter how much I enjoy seeing the creativity of people who build inns, lighthouses, and ships, none of those builds offers anything beyond decoration. Map rooms do, on all servers. Arenas also do on PVE(-C) servers, if they’re built around a battle standard.
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@Ulyssi already pointed out that your interpretation of the rules was specifically addressed in the previous discussion thread. I just want to add that your interpretation stems from taking rules out of one context and applying it to a different one. The words you quoted are in the section about “Bridges, roads, paths” and the full context is as follows:
Unless we get additional clarification from Funcom stating the contrary, then the public map rooms should not be against the rules per se. A map room could still break one of the other rules in the TOS. For (a deliberately contrived) example, if your map room design incorporates walls that enclose the obelisk, this would probably violate rules about blocking content in the game, even if those walls have gates that are always open.