I agree with you that aluminum is a premium material for rack construction but it is because of the light weight and corrosion resistance. Aluminum would actually fragment with less explosive energy than steel. Steel if fragmented as you had mentioned has more mass and so could hurt someone at a greater distance. For the purpose of fireworks racks the both steel and aluminum are very unlikely to fragment unless the rack design encloses the mortars creating containment very close to and around the mortar where the shell sits in the mortar. This would go against any sensible rack design. Everyone who has enough knowledge to design a safe rack knows that it is a key feature to minimize containment around the mortar especially around the portion of the mortar where the loaded shell sits. So unless it is a very bad rack design fragmentation is not a problem for both aluminum and steel racks. Below is a picture of an ATF type rack that was built by someone that did not understand the importance of minimizing the containment around the mortar where the shell sits. There was no fragmentation of the aluminum but I would not have wanted to be in the way of the fragments of the mortar and the HDPE rings that hold the mortars.