Those that disagree, what are your reasons for that decision?

If someone really, really wanted to trash talk and be abusive they could still do so in party or trash talk behind their back in company or linkshell channels.

It is a focus issue? Everyone in the dungeon should focus on the task at hand instead of chatting it up. Company chat and linkshells are still available though, so exclusion of /tell is ineffective.

RMT ads clogging up your chatspace or distracting you from your duties? That is understandable but I don't see why /tell should be prohibited for such a reason.

Do you find it a waste of dev time to address such a small issue?

Or are you disagreeing out of spite?