It's an interesting approach. It is similar to the one suggested in a similar discussion recently about the CSMIO/IP-M, which now has a slave output for dual-motor setups but no squaring capability. In that case, the suggestion was to use something like an Arduino processor alongside the motion controller, driving the stepper drivers by combining the two sets of outputs (from motion controller and Arduino). The Arduino would pick up limit switch signals and be able to execute the homing/squaring sequence, but apart from homing, would do nothing. The "Planet" solution sits in line between motion controller and stepper drivers so in that sense is a bit more intrusive, and because it uses discrete logic gates, circuit complexity limits it to managing step pulses only and the user has to set direction. More margin for error?

Personally, I would go for the Arduino just because it's much easier to set up physically and probably no more expensive (using a pre-programmed bare ATmega chip from an Arduino) and I'm thinking of exploring that when I get my new machine running using the CSMIO/IP-M but there still seems to be a bit of a debate about how much the squaring operation needs to be done, and whether you can't do it manually almost as easily.