So, does art have any tangible, measurable benefits?
Yes it does, but they're seldom the most important.
In a utilitarian view, you can imagine art as a way of taking raw materials, adding value to them through processing and assembly and delivering something whose total value is greater than the sum of its parts. While this may not be the case in all art, I think it's true in enough art to demonstrate measurable benefits at least some of the time.
For example, is your knowledge of anatomy and skill in sculpture good enough to produce something as accurate as Michelangelo's
David? Mine is not. Yet in studying
David I can improve my knowledge of both anatomy and sculpting. I can calculate the equivalent cost of growing this knowledge some other way (say, through a $10 book), and multiply it by the number of students who study this sculpture (say, 100,000 per year), and that gives me a lower bound on
David's nett worth (say, $1,000,000 per year - whether or not any charges $10 to see the sculpture). Bear in mind that
David is now 500 years old. It cost Michelangelo only 4 years of his life to make (far less than the human development cost of the first version of MS Word say), and it has more than repaid the investment in his time as a didactic tool (will the current version of MS Word last 500 years, d'ye think?

)
But I don't believe that this is
David's real material value. I believe that its real material value is more closely associated with the decisions I make about myself, my society, my relationships based on what I know. If it happens that
David teaches me something important about
these things, that can completely change the character of my decisions, and those decisions propagate and multiply over time in social and human capital. Here's an illustration.
I have been a professor and presently run a consulting company. Suppose that I believe that young men are worthless to educate, train and manage (I don't believe this but some teachers and managers
do). Suppose that after viewing
David, I then see the nobility and vulnerability in the sculpture as an exemplar of young men, and this makes me realise that I need to nurture young men for tomorrow. Suppose that I bring that insight and belief back to my work -- in other words,
David transforms my opinions and perceptions -- a quality for which art is renowned.
In the course of my working life, I might touch the lives of hundreds or thousands of young men, and if I am changed from being contemptuous of young men to being responsible for them then that is the number of young men who may prosper under my teaching or management. They in turn may affect hundreds or thousands of young men each themselves... Over time this produces a nett social impact that even MS Word cannot rival, and all because a talented artist 500 years ago chose to make a sympathetic study in male youth.
This is why I believe that it's the transformative benefits of art that are the most valuable. Art can make arguments and deliver insights that you can't find anywhere else, and they can change us forever.
Of course, a lot of art doesn't do this -- and the art that does isn't always a masterpiece. So in evaluating the worth of investing in art from a social capital perspective I believe that we must do it a bit like shares... You invest diversely but wisely, so that on average, the benefits far outweigh the costs.
Can art also
damage social capital? Yes, I believe that it can. I think that artists who aspire to some sense of professional ethics must consider the potential impacts of their art, and ensure there is adequate focus, appropriateness, context and balance in the work to have reasonable likelihood of delivering a positive outcome. Many artists would like to be free from accountability in expressing whatever they like "It's just art"/"It's just entertainment", but in practice I believe that the artists who
are, are also free from professional ethics.
In short, I don't think that you can have it both ways: to claim that art is valuable social capital yet somehow universally benign and free from social responsibility.