I strongly suspect that the Cambridges having Christmas at Bucklebury has more to do with the fact that Pippa and James are currently engaged than it does with the Cambridges themselves.
It's possible that Pippa and James want to spend the day together but, since the Middletons go to church at St. Mary Magdelene, they don't want to trample on the "married ins only, no fiances" rule that the royals have for Christmas. Or the Cambridges feel that James shouldn't stay at Anmer for Christmas until after the marriage.
It's also possible that the Middletons want this Christmas together before Pippa and James start alternating Christmases between the two families.
It's possible that Pippa and James want to spend the day together but, since the Middletons go to church at St. Mary Magdelene, they don't want to trample on the "married ins only, no fiances" rule that the royals have for Christmas. Or the Cambridges feel that James shouldn't stay at Anmer for Christmas until after the marriage.
It's also possible that the Middletons want this Christmas together before Pippa and James start alternating Christmases between the two families.