Tuition also entitles you to access library resources, university software, campus gyms, sports facilities, child care, and university provided health services. I think your emoticon is implying your statement is tongue in cheek, but I mean.... they absolutely do though. If you don't consider taking courses a benefit to doing a Ph.D. then yeah, the value proposition of a Ph.D. program is quite lopsided. Tax-free tuition is an amazing benefit from my perspective.
> Your industry friends could easily travel the world, and they wouldn't be stuck in a conference hotel for half their trip.
You say stuck at a conference hotel, but I love conferences. They are so much fun and I always learn so much and meet amazing people. Moreover, what I said was "travel and present research". Not many employees who have been working for 1-2 years out of college get to present their work in front of an audience of peers and experts. It's great exposure and an amazing way to make connections and start building a career. My friends in industry were never given opportunities like that.
> paying them entitles you to the US social safety net!
I mean, you start paying into them soon enough -- I'm not really worried about my social security payout when I retire because I didn't pay into FICA when I was 24. The benefit of not paying FICA is that $30k goes further now when you need it. With Obamacare, healthcare isn't much of an issue anymore for grad students, and we got cheap/free healthcare at the university health services.
Look, all I'm saying is that when you say "$30k with marginal benefits" it makes it sound like a grad student is being compensated as a fast food employee. It's more like: $30k, and there are all kinds of benefits which may or may not appeal to you as an individual. If you want to make a ton of money go into industry. If you want a different experience do a Ph.D. You will (should!) come out of it with an education and a lot of open doors, some of which are permanently locked or vastly harder to open without the Ph.D. degree.