Found an interesting reprap thread, where a hybrid delta-cartesian bot was proposed. It uses only 2-arms to drive X/Z, and a table for Y axis. I could see this at the very least removing the need for dual Z motors. Check out the pictures in the thread.
http://forums.reprap.org/read.php?178,374352
Great find!
This idea has been proposed a couple of times in the last few years, though I think this type of mechanism makes more sense in the X/Y plane (with a Z platform) than as shown. Like the way delta printers are usually made from linear deltas instead of rotational, this would be a linear version of the SCARA mechanism used by reprap Morgan. Another variation of this, which I’ve also seen proposed several times, would be to do this with the two carriages on a common rail, though I think that using separate rails as in this example would result in a more desirable build area geometry (at the expense of the “openness” of the single-rail version).