The real move here is to roll them out in fields mostly unusable for other purposes. These days, you can put up an entire field installation in a day or two. Getting them onto roofs requires a whole lot more effort for a whole lot less capacity.
Furthermore you can take a shortcut by mounting them either flat on the ground (dead simple) or vertically (no worries about dust or snow) (use them as a fence). The loss of efficiency might be marginal, depending on your latitude and weather.
