Bearing loading and drag.
if each bearing produces drag, then 8 produce more than one, then 16 produce more drag especially on quads. A perfectly spaced set of bearing in a quad wheel means that two bearings would share the side loads, producing more drag than one bearing alone that is taking the side load. By what percentage of increased drag, unknown. The unloaded bearing would still add to drag but I would think at a lower percentage.
Inlines, all use spacers. Most of their loadings is vertical with some but very low sideloading. Bigger wheels, less rotation, less overall drag.
Quads skates load bearings both vertically and side load. Imo, a skate set up without spacers reduces side load drag in turns and would perform as well as spaced setups.
Bearings are only a small part of a system pieced together from selected parts. Still the most precise of the skate. I've seen good skaters spend big money on bearings and go no faster than before, then change wheels and pick up noticeable speed in minutes. Some racers save their wheels by not using them unless racing from year to year. Skate weight is also a big factor.