If we were able to use tells, we would not look warily or attack players not of our race. We would get talk about Joe the Demon being friendly instead of a mass attack against the demon. Tells are one of the most abused features I have noticed in EverQuest. I never had a moment's peace with my druid or wizard. Removing tells removes a large part of the annoyance players have to deal with when they have skills that are in demand.
Tells destroy the purpose of different languages.
No longer will they have to learn a language, which cuts into monster killing time. All they would have to do is tell the guy who didn't understand Giant to get the message across. A tell system creates a global economy. In an RPG, economies are supposed to be town based. If tells were implemented, everyone would know where to buy the best equipment, and if the need be, tell a friend to go into the town to get it.