[The Fifth Horseman]

At first AB and AR had the same group of people forming their core communities, but things have changed a lot since then. Some members (and a few of the staff) have moved over to AR completely, while others only check the additions at AR once in a while and don't bother with it much otherwise.
As each site has different focusm, it also attracts a different type of community - so the difference continues to grow more pronounced with time. Merging the sites now would cause a total culture shock to both communities, one they might never recover from.

Merging them is a bad-BAD-BAD idea.