I understand why escorts do it. They attract far more nutters than the rest of us do, and are easier to find. There are some very limited circumstances in which it makes sense for staff or mods. If, for example, they only have time to log in quickly and so aren't able to respond to pm's from people who need something done urgently then it's better if those pm's go to a staff member or mod who has time to deal with them. Most of the time, however, staff and mods should be visible. And I've never felt any need to be invisible as a punter.