For some kids, staying with agemates is really important. Sports, Scholarships and Academic Comeptitions come to mind as potential reasons to avoid full grade skips.
I agree that personality plays a large role. Learning pace is key. I think that LOG is key in that it shapes so much of how people respond to a kid. In my son's first school, they just couldn't see the difference between him and their handful of bright kids, except that he was twitchier and crankier and less willing to do things their way.
Some kids will find other interests. Some schools will 'see' HG kids for who they are. Some schools will offer more flexibility. Some parents will demand afterschooling right from the begining. Some people are naturally more easy-going than others.
I think the luck of finding a friend makes a huge difference - or a mentor or a favorite teacher. I think the biggest benifit to my son with the grade skip is the chance to develop his work ethic, but the social aspects are a close second.
((Picture a 6th grade kid, who walked around the house saying:I'm so sad. None of my classmates can name all 4 Beatles. Most of my classmates can't name one Bealtes song))
The older kids just seem to know more of the world (I'm not suggesting that Beatles-knowledge correlates with intelligence) and 'get' his jokes. One of the nice things about High School is that the grade levels mix more freely.
Just some thoughts,
Grinity