I don't NEED purebreds at all - but I prefer to know the guidelines of what my dog will be like as an adult. We also will not adopt from the RSPCA due to my mum's dog almost dying in there from the terrible care, so we don't really have much choice. My puppy is a Lab/Poodle mix (parents bought her) and it is so much harder with her because of the mixed attitude. Nobody NEEDS purebreds, but some people may prefer having something they know will suit you. If you got a total mix, you'd have no clue how big it could get, its exercise levels, shedding, etc.. With purebreds, it's easy enough to tell.
FYI : Not all purebreds come from BYBs yano.