But there isn't one, because no-one that's had it done cares, usually they are glad they have pierced ears but no pain to remember
You know all of this, do you? 
It is an ethical point. It's not about 'did you mind having your ears pierced? No? Everyone STFU then.'
Morphene explains the issue really well.
Does a baby's body belong to the baby or the parent. If you believe in respecting a baby as a whole independent person then it is inconceivable to decide to pierce their ears
So whether or not its trendy, cultural, painful or not, whether you think it is harmful or worth the risk of infection or not, the really important point is do you respect your baby's body as their and not yours to modify
That is way the debate is heated - because its the ethical line that matters, not the severity of harm done