I had a discussion the other day with my office, I feel weather tight would meet the code. (existing install is Cls2-div1).
Coal power plant, dock side open conveyer for unloading the barge, Only a shroud over the belt.
My scope: Fire alarm pull stations,120V AC onthe outside railing,notification only to control room. Deluge is manual operation.
The railing is 8' away from the conveyer system,in the wide open air,along the river. All conduit is threaded rigid.
I feel weather tight would be enough.

For the most part, you're correct. There are two situations that bear considering, though:
1) Where there are moving parts. For example, you don't want dust to get within a breaker, and prevent it from operating. There you would be looking for a dust-tight enclosure; and,
2) Where heat is generated. Motors and light fixtures must remain cool enough that they do not ignite the dusts ... even when they are buried in the dust.