Regardless of the good intentions behind parents or parenting experts that do not punish, it does not set your children up properly for life in the real world.
Child hits - no punishment
Adult hits - police involvement, fines to pay, perhaps prison time.
Child steals - no punishment
Adult steals - police involvement, fines to pay, perhaps prison time.
Child is mean, rude, violent etc. at home to parents - no punishment
Child is mean, rude, violent etc. to other children - they are punished through school discipline measures or by other children not associating with them.
With my Dds I try to have a reasoned discussion the first time they exhibit an undesirable behaviour. If that behaviour is repeated, then a consequence is given. Consequences will get more severe if the behaviour is continually repeated, but luckily I never really get to that stage with the kids.
For adults, all behaviour, good or bad, has a real world consequence. Children need to be taught that.
This final point is offered as a kind of devil's advocate question.
If you decide not to punish poor behaviour, would you consider a parenting model where you don't reward good behaviour either?