It would seem to me that much of ATI's interest in XGI would be the same as nVidia's interest in ULI...not primarily the technology, but the local business presence and network....
...When you are becoming more and more of a global company like ATI, at some point in your growth you try to localize parts of your business. Particularly, you have local sales offices that network with prospective customers. It is very difficult to "grow" such an infrastructure when you are foreign to that market / culture.
...I'm sure there is significant value in the engineering team (as geo suggested) as well. But I doubt that the technology in and of itself was a major driver for the acquisition. (Assuming this is all true, of course!)