Should I rinse canned beans before using them?
I finally found what I'm looking for, from the University of Michigan - some actual data on the subject!

They say that rinsing canned beans can reduce the amount of sodium by half, and also reduces the amount of complex sugars which humans can't digest (but the bacteria in our intestines can, with uncomfortable results!)

It appears that draining the fluid is likely to improve the flavor and texture of the resulting food by concentrating the flavor of the beans -- unless you're following a soup recipe that specifically suggests retaining the liquid to thicken the soup.

I still haven't found any information about what healthy nutrients might be lost by rinsing the beans, but the general consensus seems to be that it rinsing will do more good than harm in almost all cases.