As a native and resident of Watauga County, NC, it seems like a terrible choice as a bellwether. Using PVI and the fact that it went for Obama in 2008 and Romney 2012 ignores the county's unique demographic dynamics. The county skews incredibly young and incredibly white. College students make up around a third of population. Clinton clearly isn't inspiring the youth turnout like Obama. If you remove college students from the equation, the county is probably near 99% white. I will also note that the local GOP has been active in suppressing the youth vote by attempting to limit early voting and closing polling sites on the campus of Appalachian State. I could easily see a situation in which the county trends R, but the state of North Carolina trends D.
It is Watauga County's not-so-unique demographics that make it a bellwether. Quite a few counties with mid-sized universities make it onto the bellwether list because the youth vote offsets the rest of the county. You don't need to have a precise racial breakdown identical to the state to predict the state average. In fact, due to racially polarized voting, those counties might not be the most representative, anyway.
Wautuga isn't the best bellwether for North Carolina due to its trend from 2008 to 2012, but it is one. Other counties had smaller trends, but some are also smaller than Watauga, perhaps making them harder to poll.
