I think it's inarguable that mediums offer a form of counselling to the bereaved - what is in question is whether their particular brand of counselling is beneficial, destructive or simply harmless in the long-term. It would be interesting to have a study done to see if those who visit mediums in the hope of maintaining contact with their loved ones find the grieving process more difficult to work through compared to those who visit qualified grief counsellors or who work through their grief unaided by outside agencies.
I am predisposed to believe that mediumship is pernicious as it holds up the grieving process. Trying to talk to the dead as if they were still living and concerned with earthly problems seems to be to be the equivalent of maintaining a 'shrine' to the loved one and refusing to accept the loss and move on and accept that person is not in your life anymore. There's also the issue of fraud - even if it is beneficial to a few people, the fact that the benefit is based on a lie, and moreover a lie told for great personal profit in some cases, to me cancels out any perceived benefit in the long run.