The "expand a computer by adding slices" concept wasn't exactly an original one when the FROGpeople were playing with it, and several designs (involving both horizontal and vertical stacking) actually went to market. This picture probably best sums up why people came to their senses regarding sidecar-based modular expansion:



Look at the IBM PCjr for another horizontal example. Using IBM-branded sidecars to add 512k and a parallel port would nearly double the width of the system, making it occupy more desk space than its older brother PC which could hold all that plus a second disk drive internally. Yay?