I think that, for people who believe someone who lived in the nineteenth century wrote the Book of Mormon, Mary being "white" makes all the sense in the world.
But how do believers explain this? Nephi wouldn't have been "white", and neither would have Mary, and the connotation "white" wouldn't have been emphasized in a community of darker skinned mid-eastern peoples. Why would someone who was darker skinned himself, and grew up surrounded by darker skinned peoples, see a darker skinned woman in a vision and declare her "white"?
We all know that other "whites" were changed to "pure". Here's Royal Skousen's explanation of why:
The third one is this 'white and delightsome' changed to 'pure and delightsome.' There has been more ink on this one than any one and it's all about modes and it's really, I think, an embarrassment. First of all the textual evidence, this change appears in the 1840 edition, we presume that it was made by Joseph Smith but we can't be sure, it just appears. It is probably not a typo, a misreading, because 'white' and 'pure' look so different, it probably was consciously done.
In preparing the 1981 edition, the Committee at the Church considered this reading in the 1840 edition and they made the change based on the 1840 edition. It is in my mind quite clear that there was no political motivation; they were not trying to remove racism from the Book of Mormon text. The reason this is very clear is there are eight other passages they did not touch which quite clearly could be identified as making the same preference for white skin and those are not removed. The brethren, I presume, and they've never said why, but... looked at the actual text of the 1840, decided that it was Joseph Smith's change--I think it probably was--and decided to follow that. It was a very conservative change and could not have really been motivated by political considerations. As far... we don't have the original manuscript here, the internal evidence suggests that 'white' is the original reading and it is because the word 'white' co-occurs with 'fair and delightsome' and refers with reference to skin color six times in the text. 'White' co-occurs with 'pure,' but only when referring to a state of heavenly perfection like in the resurrection; that's four times. So we don't know why the change was made by Joseph Smith. The 1981 was probably made out of deference to that change that they assumed that Joseph had made.
http://www.fairlds.org/FAIR_Conferences ... ormon.html
This seems a concession that racism was the impetus for the emphasis on white skin color, but I'm assuming that, as a believer, Skousen believes the racism was in the Nephites. This really doesn't make sense, given the fact that Nephites would also have had darker skin in the first place. In fact, would someone of Israelite descent look THAT much lighter than someone of Mesoamerican descent?