Skateboard wheels have two sets of four wheels on each corner of the dolly. For each point, there are four on the outside and four on the inside.
The reason pros do this is to distribute the weight from 4 wheels onto 36. This has a couple of effects:
1) It helps to eliminate any bumps in the track (especially between joints) because instead of all the weight coming down on one wheel when it crosses the line between track peices, (potentially hitting a gap and causes the camera to slightly jolt.) At any given point while the skateboard wheels are crossing the joint there are at least two wheels on either side of the track, insuring stability.
2) Pro dollys, like the fishers and panthers, have pnumatic tires. When you have all the weight of a camera package, an operator and the ac on a dolly it puts several hundred pounds of weight on the wheels. With pnumatic tires, the more weight you put on them the harder the dolly is to push. So with skateboard wheels, you eliminate the problem of pushing several hundred pounds over narrow track with pnumatic tires. This makes it easier for the dolly grip as well as being mechanically better for the dolly. (I worked on a shoot once where skateboard wheels weren't used, and a tire from a fisher 11 completely deflated because of the rather large girth of the DP.)
