We had this argument before, I have to repeat myself: why not? And what's the point of large-scale installations? I won't repeat all the detailed arguments, instead, here is something new for ET interest:

Madrid, 23 June 2010
Roofs could technically generate up to 40% of EU's electricity demand by 2020
With a total ground floor area over 22,000 km2, 40% of all building roofs and 15% of all facades in EU 27 are suited for PV applications. This means that over 1,500 GWp of PV could technically be installed in Europe which would generate annually about 1,400TWh, representing 40% of the total electricity demand by 2020.