Very recently I wrote about a few peculiar 3D printer designs that you don't see very often. They included SCARA arms, and delta printers, but it wasn't immediately evident why someone would choose these designs. Sure, they look cool, but why are they better than the simple ones we often see?
Well, I was lucky enough to bump into Nick Seward at the Maker Faire as he was showing off a couple of interesting printers. He was nice enough to answer the question clearly. His response boils down to one thing: cost. As Nick says, a single linear rail system costs roughly the same as all the printed mechanical parts of the Reprap Simpson, which is the yellow one in the video. That cost could surely be impacted more if it were to do a production run of injection molded parts.
Another cool aspect of these printers is the fact that they can print their own upgrades. While all 3D printers can print parts that may improve their performance, these can actually print 20 percent larger versions of themselves! All you would have to do at that point is transplant your motors and electronics and you've got a bigger printer!