Like others have said, when I get one with a funky kink in it, I'll usually just hang it on my saddle as a "spare" for when I need to rope and drag something really heavy or drag something into a trailer (that plays hell on a good rope).
Sometimes the stretching unkinks it and sometimes it doesn't. If it doesn't, I'll normally take one strand out of it (I usually buy 4-strands), cut it down to 6-8 foot pieces, and use them as piggin strings for tying down cattle, leading dogs, and anything else I might ever need a spare length of rope for.
As for used ropes...I generally don't buy them. If I do, I don't use them for roping. I normally strip them down for spare piggin strings or lead ropes or something.