So, what's the wisdom in spending $30 an acre now when $3 will work till it doesn't? $30 today won't make his $30 later any cheaper or work any better.

Wisdom?  Well, the lack of wisdom by people running nothing but a cheap glyphosate program is why thousands of us now battle weeds which are vastly harder to deal with.  And, see, here's the deal when the $3 plan quits working:   You won't know it quit working for a week or two after spraying.  Then all of a sudden you will notice weeds popping up above the beans, and then you will be spending $30.  But the weeds will be highly agitated yet proud of themselves for growing out of the glyphosate, so they won't take up your highly expensive next application of chemistry and cooperate by dying.  Plus, you may have dinged your bean crop at a late stage when burning them actually causes yield damage.  Now you've spent a bunch of money and frustration, and the weeds are still out their competing with your crop and cost you $100 of yield loss on top of your herbicide and application expenses.  

I guess instead of learning from what happened to millions of acres in Missouri, Illinois, Iowa, Kansas, Kentucky, Indiana, Arkansas, Texas, and lots of other places you can just carry on and not be proactive about it.  Good luck.