If you're wanting multi extrusion for different materials (i.e. soluble supports, flexibles, anything with differing extrusion temperatures. So nylon+PVA is a no go), single nozzle solutions are a bad idea. You'll end up cooking and burning the lower temp plastic in the nozzle. And yes, this method has the most wasteful purge blocks. Only really good for multi /color/ in my opinion, yes it can do multi /materials/ but it's iffy.

This leaves multi nozzle solutions, and if you're doing this you'll want a system that can lift or move the idle nozzle out of the way. Things like the CEL robox, ultimaker or best yet, the BCN3d sigma. These also waste much less filament and time when printing, and can print with different /materials/ quite happily. Lifting the idler nozzle (or better yet, parking it aside like the sigma) solves two problems. The first is oozing onto the print, the second is actually colliding with and knocking over or damaging the print because both nozzles aren't perfectly aligned. There is still the issue of lining up the hotends in X and Y (z can be done with a bed sensor).

And no, the BCN does not use a proprietary calibration sequence, they are an open source company, feel free to use their ideas, software, design etc. Yes, the z axis smooth rods are an odd choice, but I guess it experiences lower loads than X and Y and they decided to save some cost.

One thing to note about multi extrusion, is it takes much (much) longer to print an object than with single extrusion.