Durability and shock-absorption You may find your wheels wear down faster than you'd like because you skate on rough surfaces. You could opt to buy all harder wheels, but it might give you a pretty rough ride. One alternative is to replace only half of your wheels with a harder durometer, like half 78A's and half 82A's. What this mix gives you is the shock-absorption of 78A's and the durability of 82A's. Skating style Speedskaters and aggressive skaters are the ones who use mixed durometers the most.
Speedskaters do it for the hybrid durability and shock-absorption mentioned previously.
Aggressive skaters often use the anti-rocker setup where the middle two wheels are very short and hard, and the front and back wheels are tall, to make rail slides easier. With the wheel diameter there isn't a whole lot to consider. Taller wheels means more speed and the wheels last longer than shorter wheels of the same durometer. Hockey players may opt for 70-72mm wheels, to keep their center of gravity down, but still let them maintain reasonable speed during play. Speedskaters usually go for the tallest wheels in the 80 to 82mm range.