I tend to think most people who hold seances and channel famous people are quacks. However, I believe that there is much that we do not (yet) understand about our body — soul connection.
I think it's likely that if one has a deep and meaningful relationship with another person who dies, that some sort of connection remains. So, it would not surprise me at all to discover that such deep connection can result in moments of crossover at what I think of as 'thin places'.
Thin places are those places (physical and temporal) where/when the separation between the concrete world and the spiritual world become becomes very tenuous — especially during profound spiritual experiences. I think it would be okay to try to contact the dead as long as it was not for a negative purpose or using damaging/dangerous techniques.