> it's better to directly pass them the region index instead of trying to hide it. 

That implies that all user code should be change to do the looping itself *and* be aware of region indices.  I think that option ties the OPM (user) code too closely to the specific ECLIPSE models we happen to be implementing at the moment.  In particular, the notion of a region index is *very* foreign in the case of a single, analytic function.