Originated from this forum post:
The suggestion is we expose another Attached ISM Property to determine how ISM handles missing types in an applied Resource Dictionary.
If this property is set to true, ISM will throw exceptions when a Resource dictionary with an unknown type is applied.
If this property is set to false, ISM will ignore those unloaded types.
As for the default of the attached property, I'd like to hear from folks in the comments what they think.