Theoretically, pattern welding two steels allows one to obtain some of the advantages of each type. Some even posit that the merging of two types creates additional advantages beyond those either steel brings (i.e. the union is greater than the sum of the parts).
At the very least we can say that the attractive patterns ARE the additional advantage not obtained by any mono-steel.
As for the cutting qualities, many have already stated that no steel rises above the heat treatment it gets. And so long as the steels used to make a damascus billet respond favorably to the same heat treatment the union of dissimilar steels would not be functionally impaired. In other words, if the process of hardening and tempering the steels is the same, you don't undermine the qualities of either of the steels, and the resulting blade is granted the best qualities of the component steels.
Or so the theory goes.
For my own part, I use patterned steel for the aesthetic. But that doesn't means it is only useful for aesthetics. I only buy top quality billets (almost all 1084/15N20).
