Painting shadows into the texture to give the illusion of wrinkles only works as far as (like Carodan is saying) when you use a simple lighting setup and when realism is not a priority.  In any kind of closeup or in a variety of lighting setups it will become obvious for what it is (a smooth surface with shadows painted onto the texture).  It gets much worse when the mesh is morphed or animated and suddenly it's VERY obvious, because (for example) smile lines appear on the face's geometry and make shadows that do not match the shadows painted onto the texture.  This is why nearly all skin textures are not that great (shadows painted into the collarbone area, under breasts, under buttocks, various other places).

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