Why Is Ray Haffner it's hard to say, but here comes my theory:
1)His age is perfect, not too old, not too young. He's member of the visualize, which gave him the possibility to draw the smiley at the Barn while he was a teen.
2)His high fits in. 
3)The lover of Rosalind Harker (blind woman) was called Roy (Roy=Ray?) Maybe it's just a coincidence. 
4)The name RJ gave to the psychiatrist was Jay Ross (JR :D) and again Jay= Ray? could be a coincidence again. The psychiatrist affirmed that he is middle-aged, good health, and he has a good posture.
4) Why did RJ left Teresa alive? We all know that Haffner has feeling for Teresa, that could be the reason.
5) Red John must be a person, who's not successful in his real life (not like Bred Stiles), and that's the reason why he need's to create a false identity. Haffner is not consider to be a great mastermind...
6) In S04E02 when Haffner became Jane's boss, he wanted to get information from Cho, and that could have been a good plan, but he didn't knew that they were such a good team, and they will never betray each other. However this shows us that Haffner always makes tactics, and he's sober-minded. Also when Cho said that Jane is smarter than Haffner, Ray has a(n ironic) smile :)
+1 Do you think the man who is working out in such a dark place alone (S06E02) is not a psycho?

Please share with me, your opinions!