Spoiler: vertical panels can rid themselves of excess heat more efficiently, thus operating temperatures are lower and, hence, output better. Vertical panels will also be more susceptible to damage from high winds but less susceptible to precipitation of water and dust, so my guess would be that the fixed slant orientation for PV panels in temperate latitudes will stay with us irrespective. I'm starting to think about having water pipes running along the backsides of the panels to cool them and obtain warm or hot water...