This one came to my mind the other day as i was thinking the post about Deacon's character.
My point is that Mercury beeing an extra sentimental driven individual must have pussed a lot of buttons in the fields of unconscious relations within the Queen members.
If i had to over simplify the dynamics within the four i would say that:
For Brian he was a brother ( May is an only child)
For Roger the special friend ( stimulating eachother in the larger than life/out of the box expression)
For John a father/protector ( Deacon lost his father in a early age)
In John's case (Mercury the father) it may explain a little further his decision to leave because to him the Queen machine/group/family was completely headless after Freddies passing.
P.S. I know i am using imagination here but if i had to make a hard guess about his impact that would be the case.
Stelios