What evidence do we have for it? What evidence do we have against it?
Khot gave a UGC survey talk at CCC 2010. The write-up is here. The final segment (bottom of page 30) has his opinions on this question.
Another interesting survey about UGC by Khot cited in  which is more math oriented:
 S. Khot, On the Unique Games Conjecture, CCC 2010.