It looks like nsHTMLFormatConverter and nsIFormatConverter are excessive abstraction and architecture. Callers could be adapted to call nsContentUtils::ConvertToPlainText with the appropriate flags once bug 650784 has landed.
Did you check how many extensions use this? Myself, I have not seen this being used ever, it's the first time I see this, IIRC. I think people simply didn't *find* it as the API to nsPlaintextConverter and thus didn't use it, so a simple comment in the right place would do wonders. Personally, I don't think it's such a bad idea, though. I'd just combine getInputDataFlavors(); getOutputDataFlavors(); boolean canConvert(..); into getConversionMimetypes() which returns a single list of "ab/dc; e=f|gh/ij; k=l" (from|to). But I can see why you would prefer a single ConvertHTMLToTXT() function. It's much easier to implement and to use by extensions.
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