Not a fad by any means but too often done injudiciously leading to what I call SHOP 101 Design. Can look very nice and well executed when used as in the examples above - a table top or stool seat of one species and the base of another - but can go downhill on the slippery design slope real fast when, for example, a door panel is one wood and the frame is another or mixing multiple species in a table top. To my eye, this is often a bad call. I would rather mix textures than wood color. My 2 ¢, not worth even that to some.
"... for when we become in heart completely poor, we at once are the treasurers & disbursers of enormous riches."
WQJudge