The panels must be in the sun, so they'll always be getting that UV damage if they exist outside at all. May as well let them do double duty on my roof in that role.
It'll also help keep the solar thermal gain out of the house, reducing cooling costs...
I think there is no question a field is preferable if there are fields to work with. Of course, in a city that isn't much of an option and there are significant advantages to point of use generation (i.e. no need to upgrade power lines from rural solar fields to a city center).
I used to live in Boulder, and while I'm not sure exactly which field they're talking about it was not a long bike ride into farmland. I can imagine it working much better there than a bigger city.
This ignores any feed-in tariff which is declining, though there were early adopters with a locked-in rate who wouldn't have paid for electricity for years now.
In places like Colorado there is very cheap land that is not very good for farming about an hour away from Denver and it certainly seems like it would be cost + energy efficient to build out there instead of up.
Depends on what you're optimizing for. If you're considering solar purely from the angle of replacing carbon-thermal generation on the grid then sure, large arrays are more efficient, offering more room for amortization of fixed costs, more optimization for solar gain, and more room for optimal hybrid usage like this example.
However, buildings need roofs anyway, and will in turn get sun exposure. Since solar tech like tiles can take the place of traditional roofing, there are some double gains to be had there in that they're both doing the job of protection from the elements and taking otherwise mostly wasted energy and doing some work with it. Depending on how one gets into the weeds on aesthetics (like if they wanted nicer tiling anyway) the marginal extra capex of tiling may well be worth it as costs come down further. Building-solar also can help provide resiliency to grid damage, which by definition grid feeding cannot. For people in areas where they'd otherwise be running generators anyway, solar/res-wind+battery (and as BEVs take over near everyone will have an extremely sizable slab or three of battery around much of the time) can be compelling. Still more upfront, but maintenance-free for a decade or more and constantly providing some ROI (and effectively constant verification everything is working), whereas hydrocarbon generators require regular maintenance/testing which cost money and generate zero return otherwise, they just depreciate. And local solar/wind/utility resources are going to affect the time horizons for all this.
So basically there are a ton of new variables and enormously more scalability up and down the spectrum for renewables and batteries. Doing the math is in turn going to be very individual, but it will often still make sense to do both.
Also:
>why don't we do this more often?
I mean, we're still in a pretty steep part of an S-curve here. It's just plain early days. People are still experimenting with stuff like this and learning what works. Unit costs are dropping, which in turn changes what projects make sense which in turn changes demand and thus unit costs. Grids are adapting and getting smarter. Both storage and opportunistic demand are doing the same in parallel in a variety of ways. Stories like this where someone tries some new stuff and it works out well will make others perk up and take notice. There will also be things tried that don't work out. Going to be a wild decade.
On the other hand you also have the phenomenon that after a person puts solar power on top of their little detached single-family house they start yammering about "solar access rights" to stop the construction of even slightly taller buildings nearby. Boulder, Colorado is ground zero for this kind of stupidity, see their "solar access protection" law which is as naked an act of NIMBY greenwashing as anyone has ever seen.
https://www-static.bouldercolorado.gov/docs/PDS/forms/815_So...
Naturally, that power output will differ by location, but it's doable.
[0] Based on my local area, this is a mix of row-houses, semi-detached, and fully detached housing.