I love them...they're highly textural and can be wonderful additions to mixed media. I've done the first example with HB pencil and a wide range of textured items. Repeated light brushes of the pencil across a paper laid on a textured surface can produce a wide range of pattern.
Some things you might consider attempting rubbings on: stencils, textured doilies, plastered walls (truly lovely!), a row of books with spines aligned, stamps, bricks, textured wallpaper, a collagraph, and so on, and on.....and on. Happy frottaging!