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At least we need a chance to set parameters like "importance" and "nsamples" for each defined arealight?

  Maybe it's not easy to decide if a geometry with ambient != 0 is an arealight or not. 

I'd guess that BB already put some thoughts into that for his material converter.

Anyway, for me it would make more sense to add parameters to the material and not to a prop.
That'll probably look odd, as it would need to be done via additional (unconnected) nodes (probably distinguished by unique names).
Otherwise you'd have parameters in the prop's properties (probably multiplied by the number of material zones) and material settings per material. So you need to check materials in the properties palette and in the material room... And you cannot just save material settings to the library and reuse those materials because the setting are in the prop's parameters...

But that's just my two cents :-)