Disclaimer: Don't own... there would have been much more McCoy if I did.
Please excuse any spelling errors.
Bones was not the sort of person Jim Kirk wanted at his back in a fist fight. Not because he didn't trust him; Jim trusted Bones with his very life. Or because he wasn't any good; The man had a mean left hook when he did fight. But because he always looked a little green round the gills when it was over, and for days afterward he would be steeped in self recriminaton, and Jim could not stand to see his friend like that, espically when it was his fault it was there.
Jim Kirk knew that the truth of course, and Bones' reluctance to fight was only partly because he was a doctor and was, as such, very non violent. He was the only person who knew that Bones' freezing as the fight escalated from shouting to blows, had nothing to do with his feeling as a doctor, and everything to do with the memories of the divorce, both his own and his fathers.
Bones was a world class verbal fighter though, and if there was ever to be a battle of words Jim would drag Bones into it with no reservations at all, but if it looked like it was about to turn physcial he was the first to get him out of the way. It was better for Bone's to have to patch him up after a fight, than it was to watch him rehash memories he clearly didn't want to have.
Which was why Jim didn't hold any sort of grudge when Bones didn't come riding to his rescue during either confrontation he'd had with Spock, he knew his friend well enough to forgive him, he knew that if either fight hadn't been over in a matter of seconds Bones would have come to his aid.
Bones could hold his own in a fight if push came to shove, as long as the person he was fighting wasn't someone he considered himself to be close to, or responisable for, in which case Bones seemed hard pushed to even defend himself. Jim had learnt this more by accident than anything else, and though he couldn't be entirely sure of all the details involved he remembered that clearly enough.
All this of course was something that should immeadiately veto Bones' inclusion in any landing parties, along with his Aviaphobia, which he had never really conquered, and his nervousness around the transporters. But Jim always made sure that Bones' name was right at the top, under his own and Spocks, because despite his claims of 'Dammit Jim, I'm a Doctor, not some hotshot explorer with a deathwish!', Bones was his best friend, and it just wouldn't feel right going down to a planets surface without him.
Bones wasn't someone Jim Kirk wanted at his back in a fist fight, but he'd be damned if he didn't want him there for everything else.
This was written mostly because I'm very bored, and because I noticed that Bones didn't come to Jim's aid during the film when he got into fights, and because I like the idea that Jim already understands why he didn't and can forgive him. Because McCoy is the best character ever!