Good question but just raises another for me - when would you use NULLIF? It may be useful, but I am struggling to see where you would use it. Why would you want something to return NULL if two values are equal? Is it just easier than using a CASE statement?
MARCUS. Why dost thou laugh? It fits not with this hour.
TITUS. Why, I have not another tear to shed;
--Titus Andronicus, William Shakespeare