Many people who first venture out into the blogosphere do so under the cover of pseudonymity, even if they later blog under their own name. There are good pseudonymous bloggers who really are in positions that make it so that they would not blog at all if they had no such protection. If those protections don't exist, if we do not protect the pseudonymity of others, that contributes to an atmosphere of hostility in the blogosphere, many good bloggers will be lost, and we will all be the poorer.
When considering these sorts of cases, people often seem to end up only focusing on the particular case at hand, and so determine whether the 'outing' was justified by assessing whether they dislike the outed blogger enough to trump his putative privacy rights. But it's also important to consider the broader impact, as Brandon highlights in the above quote.
We should want to uphold norms that enable pseudonymous blogging. The most straightforward candidate norm would be to respect a blogger's pseudonymity unless there's some pressing reason why the public needs to know their true identity (e.g. to expose sockpuppetry, undisclosed conflicts of interest, etc.)
One might propose a less accommodating norm, e.g. to respect pseudonymity only insofar as the blogger remains civil and inoffensive, and expose them if they piss you off. This may be motivated by the idea that if people want to engage in verbal attacks, they should have to own their words, rather than hide behind the veil of their virtual persona. Or, if retributivism isn't your thing, perhaps such a rule would have the happy consequence of reducing the amount of vitriol that gets thrown around online. People can still feel safe blogging under a pseudonym; they simply need to take care not to be jerks while they're at it.
The obvious problem with this proposal is that "offensiveness" is rather subjective. Is maintaining a polite tone sufficient to guarantee your pseudonymity, or might an opposing partisan decide that your views are substantively offensive, 'beyond the pale', and that you "deserve" to suffer real-life censure for them? If we grant everyone too much discretion in determining whether or not another's pseudonymity ought to be respected, someone is sure to judge poorly, so we'll end up with much weaker protections than originally intended.
So I think we're probably better off sticking with the straightforward norm to basically always respect pseudonymity (except in the very rare and uncontroversial cases of sockpuppetry and the like).