Because all players would let /tells interrupt their runs, therefore block all players and remove all choices. Right.
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Because all players would let /tells interrupt their runs, therefore block all players and remove all choices. Right.
This is a lot more likely to happen with FC/LS chat (since that's where most chatting takes place) than with tells.
I think everyone is overthinking possible reasons based on what chat players would (or wouldn't) want to see, when the more obvious one is simply that dungeons are cross-server.
Names are only unique on individual servers. The "/tell" command cannot identify which player you're talking to if there are multiple players with the same character name. In order for /tell to work in duties, they'd have to change the format from "/tell <player name> <message>" to "/tell <server>:<player name> <message>", and then they'd have to also update the party/alliance windows to show which server everyone is from.
I could deal with the change in the /tell command format, but if they're going to clutter up the screen by adding more info to the party window, I'd rather it be info that's more directly useful to completing the dungeon (like a TP bar).