Well, one could make a similar argument about the splitting of Dorchester into North and South, which (Charles, correct me here) is a legacy of the bad old segregationist days when the city arbitrarily split the neighborhood map to make clear where the "good" people lived.

Or one could realize that, perhaps, one is reading way too much into what is basically a good poster/t-shirt idea. I lived in Brighton for 12 years, so I can tell you roughly, where Brighton ends and Allston begins on Allston Street, but I never once got upset at somebody referring to "Allston/Brighton" or even, as this maps puts it, "Brighton/Allston" (see the Brighton Allston Historic Society, which sort of makes sense given that Allston grew out of Brighton).