Also note that in a few cases a grey value for metalness is OK and should be considered, e.g. if your material is a copper patina it will probably be somewhere in between. But don't quote me on that! :p
Generally rust, patina, any sort of heavily oxidized metal can be represented as a non-metal, and shouldn't use grey values. A transition area between raw metals and oxidization makes sense, but rust etc can easily be considered as a non-metal, think of it more as "used to be metal".
I think the only real difference is I'm talking about it in terms of materials, ie: X pixel is A material and Y pixel is B material, while you're talking specifically about a certain effect. Depending on the effect, the transition area may be wider or smaller, or stronger/weaker, though generally even a light coating of…