So here I am looking at this huge oracle 10g table. I looked at its indexes and see that ALL of the columns are under one unique index. Does this provide actually provide any performance benefits?
Possibly, possibly not. It could be that the unique index is implementing a constraint to ensure that rows are indeed unique, and is not intended to help with performance at all. There could be a performance benefit for indexed lookup queries, because they won't need to access the actual table at all.
On the face of it it sounds like this should have been created as an INDEX ORGANIZED table.