This is merely my personal opinion.
- The transaction was too important for Hendricks so he had to do it himself instead of sending Wistrom or any other henchman.
- Hendricks is wanted and a lot of people are looking for him for his extreme ideas including Ethan and his teammates. So he can't possibly travel from Russia to Dubai off the grid.
Thus, the disguise was the perfect solution for this. And who's better to impersonate than your right hand man Wistrom.
As why he revealed his identity to Ethan, I think to enjoy a moment of victory over him. And I haven't watched it for a while now but I think the mask was already half ripped off by the sand storm and the fight they had, so Ethan may already deduced that he was Hendricks.