It seems like a lot of people know that keeping the hobbits all white is a good idea but can't quite reason it out. First off, fantasy shouldn't be held to a lower standard, just because its make believe doesn't make it a lower form of art. In fact, the best fantasies all make sense in their own world, there is a reason fans get angry when a fictional character does or is something unreasonable, like say how Aizen from Bleach turned out to be (spoiler alert) unbelievably powerful pissed off lots of fans. That being said.
I don't believe the hobbits should be just white cause that's what the author said, but because to give them a multi culture look would ruin (imo) the most important thing they bring to the story... Innocents. They are a sheltered race that has had peace and happiness even though the outside world has been undergoing war after war, basically, they are inbreeds. It doesn't make any sense to have hobbits of different skin tones because they aren't multi culture, they are the opposite, they have been living together in peace for so long they all look the same and act the same. Even if you did write it into the fantasy world for there to be darker skinned hobbits, it doesn't change the fact that the viewer would associate the differences in skin color with different cultures, again ruining the sheltered hobbits idea by making them seems more worldly.