@Tim, Niall, some basic wind facts
Tim Spence: The energy in the wind scales as the cube of the wind speed, so you wouldn't just need to go from 1 generator to 2 to 3, but from 1 to 4 to 9 and so on. The hub and gearbox to handle 9 generators worth of torque would be so bulky and have so much resistance that at normal (1 generator) wind speed, they'd eat your efficiency. Wind machines are designed for a specific range of speeds. Below that, there's not enough energy in the wind for them to bother with, and above that, there's so much it's nearly impossible to capture.
Niall: Vertical axis wind turbines have a tougher time than horizontal-axis machines, for one simple reason: On each revolution, every blade goes from facing into the wind, to facing against it, to facing away from it, to facing against it, and back into it again. This creates oscillating stresses on each blade, which are much harder to withstand than the constant stress experienced by the blades of a horizontal-axis turbine.
And yes, the force of gravity does technically oscillate, relative to the blades of a horizontal axis turbine, but gravity is minor compared to the other forces at play in such a machine. Proponents of vertical-axis machines seem convinced that a giant conspiracy, not the laws of physics, is to blame for the thousand-year dominance of horizontal-axis turbines.