When it isn’t necessary, a blanket Division 2 classification, “just to be safe,” is usually an exceptionally expensive proposition and often the result of a design from ignorance. (This is not to say the designer is stupid – they just weren’t quite sure what they were doing.)
In a large outdoor installation there may be some justification for a blanket classification because it does indeed reduce the number of boundary seals required. However, where walls may present substantially different pressure differentials you really need to have a documented boundary. It only takes a differential of 6” of water and many HVAC systems can create that.
RMC isn’t the only acceptable wiring method, See Section 501.10. Be careful when thinking about drop cords. See 501.130(B). Luminaries are not usually required to be explosionproof, but they will need to be specifically identified for Class, Division, Group and T-rating.
Robert B. Alexander, P.E.
"I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means." (Corollary to Charlie's Rule)
Answers based on 2014 NEC unless otherwise noted.