The original reason given for preventing tells within a duty was because they were concerned people would be abusive toward newer players.

Unfortunately it also prevents you from helping newer players without doing it publicly.

For instance, if I'm in a dungeon with a new tank that doesn't quite know what he's doing and I want to help him out...
- You know if you Shield Lob one of the targets, you'll have better threat than just walking up to them
- You might want to Flash a couple times to pick up everything, the healer's getting punched a little more than is necessary

That sort of thing. I have to do that in party chat, which can be embarrassing for a new tank. Embarrassment can lead to a defensive response rather than lead to him learning to do a better job. People don't like being called out in public, and people generally don't like having their lack of experience pointed out. It's MUCH better if that can be done privately, just between me and him.