I agree, effeciency is only a concern as a function of space available. If you have a flat roof then all the space is available, and depending on your latitude

I agree, effeciency is only a concern as a function of space available. If you have a flat roof then all the space is available, and depending on your latitude flat can be a good orientation, especially in summer. Pompei roofs do not appear to be flat, but they do seem to be very low pitch, and the panels are curved, so the whole roof is likely in play. Back to your main point, effeciency in itself doesn't matter. If you have enough space, you just install more. That drives up the installation cost though, and things like cleaning them etc. Of course ultimately everything is space constrained, but generating 11% is better than 0%.