What's the advantage of two z motors?
There are three main options here for Mendel style ZX gantries:


One Z screw and motor, which is similar to a cantilevered design but somewhat more stable because of the opposite smooth rod
Two Z screws and two motors
Two Z screws and one motor, with belt synchronization of the two sides


Of all of these, running two screws off one motor is clearly superior in reliability and user-friendliness. There is no risk of the two sides of the Z stage going out of sync. One motor running at higher current will generally out-perform two motors splitting one driver's current via parallel wiring, because one motor with twice the torque can push much harder when one side of the gantry binds up or hits a rough spot. 

The only real downside to the single motor, double screw approach is that it requires more engineering and parts. A closed-loop timing belt must be run between the two screws, with associated pulleys, tensioner, and support bearings. In comparison, using a separate motor for each screw is very simple. It adds a stepper and a shaft coupler, but saves a lot of vitamins and design complexity.

Two-motor, two-screw solutions are lower-cost and simpler to design. That's why they're used. End of story.

One-motor, one-screw Mendel style printers are quite rare. The passive side of the Z mechanism does add a little bit of stability to the X stage, but not a lot. It's possible to rack the X stage out of square with the bed and bind up the gantry. In order to work at all, they require a very wide/tall bearing footprint on the driven side to resist torque exerted on the driven side by the weight of the X stage and extruder carriage. So it's true that they don't have synchronization issues, but additional design challenges and undesirable flexure modes are introduced. It's much more common for one-screw designs to simply cantilever out the X stage, like a SmartRap or Printrbot Simple.