So... You think it's only important not to breach data as long as the person has a common name? What if that person's name is Zebediah Barrington-Smythe? Would it be less acceptable to leak that person's data? In the eyes of the law, Zebediah and Emma are equally protected.
If your car insurance company breached your data, allowing other customers to see data that could identify you, would you say "oh well never mind?"
If someone's email address was [email protected], and she had posted on the property board that she wanted advice on a house she had bought in Stoke on Trent, it wouldn't take a genius to look on Facebook and work out how many Patricia Greens there were in stoke on trent of the right age and identify her. She might have also posted about being abused by her husband, or that she's a rape survivor, or asking for advice on bullying at work, or any number of things that could cause problems if they become known. Mumsnet is supposed to be anonymous which is fine until they have another data breach.
That's why there are such strict laws about data protection. It doesn't matter why someone might misuse data. What is important is that companies like Mumsnet should protect data of individuals so that it can't be misused. It's all very well scoffing at someone who is concerned about their data being breached. But Mumsnet has a legal obligation to make sure that it does not happen and whether someone can work anything out from her email address is entirely irrelevant to the issue.