A lot of tractor trailer flatbeds and cargo box trailers have aluminum in them, on the beds etc also. Many dumptrucks depending upon "WHAT" they are hauling will be made of Aluminum also. It can and will save a reasonable amount of wt in a given rig, allow more cargo to be hauled etc.
On typical DOT class 3-5 trucks, steel wood flatbeds are in the 125 lbs per foot range, as I recall, an aluminum is 25-50 lbs lighter per foot. I could be wrong on the lbs, but still it is lighter.
As also noted, some aluminum frames etc can bend, once bent harder to fix than steel. Also it is typically softer, so other issues depending upon the what you are hauling etc should be avoided.
I would spec an aluminum flatbed on one of my trucks in a heart beat. Issues is, cost is about double$$$$$. Altho with steel right now going thru the roof. may not be as costly today as last time I priced things about 4yrs ago.
Marty