In Olympic lifting, a 1kg PR is still a PR, even if the bar weight is over 250kg. The '1-kilo rule' that came into play not too long ago did away with the 2.5lb increment (well, for them it was 1.25kg) and everything went to 1kg increments. Thats in comp, and in training. Further... this rule change came about just recently (within last decade), at a time when lifters are getting stronger and stronger... hell, even approaching, and in some cases exceeding old weight-class records (when drug testing was a complete joke). You would think if a 1kg increment was meaningless they'd be going the other way with minimum increments.
I'm curious where you got the information the best O-lifters use .5 kg plates?