<p>Some RepRap models use only a single motor for the Z axis, others use two.</p>

<p>For example, there is the <a href="http://reprap.org/wiki/3drag" rel="noreferrer">3drag</a> that has only one motor and a smooth rod on the other side. There are modifications that add a threaded rod on the other side that is connected to the motor axis with a belt - which seems to be a really good solution.</p>

<p>Other printers, like the <a href="http://reprap.org/wiki/Prusa_i3" rel="noreferrer">Prusa i3</a> or the Mendel90 have two Z motors. And after playing around with a two motor model, I find it pretty annoying when they get out of sync and I need to calibrate the axis and the print bed again. So two motors seem more like an disadvantage to me.</p>

<p>Could someone please shed some light on why most RepRaps have two Z motors (nowadays)?</p>

The general concept is to provide additional stability during the print. In the case of the 3drag machine you mentioned, you could run the risk of the -X- axis sagging due to the weight of the print head and/or additional wear on the rod or bearings on the one side (smooth side) as a result of the off balance weight. However, you may find "hacks" like
this
can potentially help reduce the affects by providing a bit more stability.
Having two -Z- axis motors with the threaded rods can help ensure stability of the -X- axis during print and can, overall, reduce wear on the mechanical components.