Use of older pieces was not done as a cost-saving measure; instead, it was meant to demonstrate that Constantine would renew Rome’s imperial past. Inclusion of pieces of monuments from Trajan, Hadrian, and Marcus Aurelius signaled to the people of Rome that Constantine regarded himself and his polices as being a continuation and reinvigoration of the reigns of those past rulers. This would have been very important for him, given the fact that his role as ruler of Rome came out of a period of bloody civil war.
The inclusion of these reliefs, unprecedented in the historical records, has also been invaluable to historians and archaeologists who have thus gained access to art and information that might otherwise have been lost. At the same time, though, the mishmash of periods produces an arch with no unifying theme or style, rendering it artistically incoherent. The only semblance of continuity comes from the fact that all faces of previous Roman emperors were reworked so that they either looked like Constantine himself or his co-emperor Licinius.
