I work in a scrap metal yard, we have scrap cars coming in all the time, some with virtually new tyres on , but may have blown engines, mot failures etc. All the tyres are taken off and then sorted through. Real crap gets chucked in a 40 yd skip and taken down to a place to be environmentally disposed of. Anything considered decent and usable is either used by pretty much all the staff at our place ( 30+ people with cars ) or sold on to part worn tyre shops.
Nothing wrong with decent part worns, not everyone can afford to buy new tyres all the time. I did cabbing for 12 years and never ever bought a new tyre.
I do however put new stuff on my Clio ..but I do use it for trackdays etc so consider it a neccessary expense.