If I lived in the Valley and had an opportunity to hang out at YC dinners in exchange for a bit of free work, there was probably a time in my life when I would've taken that opportunity.
Random other note: cool that you play the banjo! I've recently begun teaching myself the fiddle.
I need to get back to practicing the banjo. Perhaps one day we can hold a jam. The fiddle is awesome; I recommend the works of Darol Anger, and if you're near the Bay Area watch out for his concerts. Get on the Freight and Salvage mailing list if they have one. (I miss that place, although they have plenty of equivalents in Massachusetts.)
Just because some people manage to make it work doesn't mean it's not hugely discriminatory.
The thing that really galls me is that most of the time the companies that do this sort of thing are perfectly capable of paying their interns, at least well enough for them to eat and pay rent, they just choose not to because they can get away with it; there are enough college students trying to get a foot in the door and who have other support systems.