For me this sounds a bit like the "Nothing to hide nothing to fear" argument when talking about surveillance. Sometimes I don't want my behaviour to be easily trackable by others I know, especially in a profession context. For some forums I can imagine real names being useful, for instance anything which relies on the external (non-forum) reputation of the participants, e.g. a scientific discussion forum.
I can also see a case for it being optional within the same forum, e.g. A forum for health/medical advice/support might want advisors to use their real name, but give other participants the option of remaining anonymous.