I think it depends on the situation. I've had folks from the "other side" post some comments on my threads that were very insightful and heartfelt, and I appreciated them. In my opinion, if it's a comment that is helpful and heartfelt and comes from a good place, there's no harm in crossing over. But I imagine that not everyone feels the same, and there are some situations where A's want to just hear from A's and loved ones just want to hear from loved ones.