Some RepRap models use only a single motor for the Z axis, others use two.
For example, there is the 3drag 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.
Other printers, like the Prusa i3 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.
Could someone please shed some light on why most RepRaps have two Z motors (nowadays)?
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.