I think that is a wonderful addition to the scope of "prepotent" (and not considered a quibble). Like in other breeding endeavors, I think that sampling, and in some cases testing, of offspring is important.
In one scenario, two look alike parents can also be bred to other partners, perhaps specifically chosen to have a different look than them, to see how often look alikes might be produced. And so on, for other characteristics. When a cultivar is consistently associated with offspring that carry similar attributes, no matter how many other different kinds of parents were paired to the one in question to produce them, it may give some idea of the potency of the parent.
