I would venture to say a good bit. I know there's a set of them for the G* at least, and was one of the first products released for it. Similarly, when V4 was released without them, a product of this kind was made very quickly.

From a character creator's standpoint, it is considerably easier to have them part of the base mesh. That way, when creating custom head morphs (at least in Modo and Zbrush), the brows are morphed along with the head without gapping or unrealistic space between the face geometry and brow geometry, leading to a minimum of distortion. Trying to match a conformer as a completely separate piece is a considerable extra investment of time and effort, and rarely (IMHO) yields results equal in quality.

-D
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It's all fun and games until someone loses an eye texture.