Applied Engineering promised video and ethernet options for this board and its mates (TransWarp SE, 1300 and 2300). It seems plausible that they used the Radius Magicbus for the add-ons which they didn't have to make themselves.

AE were a serious engineering company so there would have been logic to any perceived madness in the design. My guess is that the spare socket was for an FPU and that the glue chips also served cache RAM on the other side of the board. Most AE designs had cache.