Greg: The only logic that I can offer is that if there was an insulation failure between the line/load conductors, there could be a loss of GFCI protection.
Some utility co's have restrictions on metered and un-metered conductors within the same enclosure; other than that and "same rated insulation" issues, that's all I heard of.
As to that statement being a part of the written word.....I have not come accross it.
