The chilling effect of insisting on real names stifles political and other controversial discussions, inhibiting people from stating their views on gun laws, feminism, terrorism, abortion, climate change and so on. When such debates are held face to face, in cafes and over dinner tables, there is little concern that, say, a future employer will learn what you said and decline to hire you (unless you have the misfortune to live in a regime with a Stasi-like network of citizen-spies), but as the internet increasingly becomes the venue of choice for such discussions, any opinion stated under your real name is trivially accessible. For anyone in a vulnerable position – people seeking a job, people whose beliefs are at odds with their neighbors or co-workers – the ability to participate in such discussions depends, effectively, on being able to do so pseudonymously.
...Online, using pseudonyms is actually more like our ordinary face-to-face experience – and it is essential for managing the impression we make. Face to face, we develop relationships in separate contexts — and the things we talk about, the jokes we make, the secrets we reveal – vary tremendously. You may share, say, your feelings about the difficulties of caring for an aging, fading parent or a special needs child with others in the same situation; you may find things funny in the company of old friends that you would never admit to thinking humorous in front of your family. You present yourself differently to your neighbor, lawyer, teacher, children, grandmother — you use different words and talk about different things. This is not a lack of integrity, but a feature of being an adaptable person in multiple social contexts, understanding the varied mores of the different situations. Pseudonyms allow us to maintain such separate contexts online.