Good call. Looking into this further, looks like you're right.[1] For the Mercator projection, translate, scale, and rotate are sufficient for the georectification of small images (read: floorplans). You'd need more of the affine toolbox if you used something like Mollweide (mostly sheer, right?), and then you might need non-affine stuff to deal with crazier projections.
I guess, then, that begs the question of why the Google floorplan submission UI uses that weird double-pane thing?
Sort of. At the level of indoor floor plans, affine transformations are just fine.