Disclaimer: I don't own anything.
Just like coffee
Yes, the Bridge is a very important part of the starship. It is a place where the most significant decisions are made, where an incompetence is not allowed, where a lapse in concentration can have really dangerous effects. Life and death of the whole crew depends on the vigilance and abilities of those who work there. It is definitely not a place for social meetings. People who have no important reason to be there, just... can't be there. There are no exceptions from that rule. At least there shouldn't be...
So how does one explain the fact that the ship's Chief Medical Officer visits this very important place so often? And, to be honest, in most cases without a good reason at all?
It's quite easy. Perhaps not everyone can understand it, but people who know the Captain can.
Because the Bridge without the presence of Leonard McCoy is like coffee without sugar. Many wouldn't mind, but James T. Kirk wouldn't be able to swallow it.