I think having interns -- but not paying them -- is exploitative and all but guarantees that young people from poor families cannot accept good placements that lead to better positions. It also devalues staff labor and undercuts intern time, when it should be treasured as a way of instilling skills and a work culture in a rising professional, and not simply a professional network. (If that.)
But times are hard, and I can imagine where an unpaid internship -- not an endless one, and even for someone not in college -- can provide a hiring advantage that can mean better pay later, or even just a boost of morale and that networking opportunity in a low period. In this sense, as occupational volunteerism, I can cope with (if not like) unpaid internships. (But if there's money, there should be compensation.)
This has limited applicability to Unitarian Universalists, who as far as I know have paid ministerial internships, if often poorly paid. But they are also too few. With these caveats, I look with appreciation at a blog post by Abigail Collazo at Left Standing Up that suggests value that can be added to unpaid internships. Something to keep in mind, too, with dedicated, regular volunteers. And perhaps, too, for short pre-seminary internships that may be useful for someone discerning a vocation.