Tone policing is the term used for when an argument is rebutted by attacking its delivery style instead of its content. "Calm down, dear" is a form of tone policing. The reason that tone policing itself is so consistently flagged and criticised is because it is often used to derail an argument away from the points being made and towards the (technically irrelevant) tone of the critic. It's a frustrating tactic and often a cowardly method used by people who are happy to stoke the fire with pointed opinions but who cannot handle the inevitable flames. Most of the time tone policing is just a bad defence and, to the initiated, draws a spotlight of weakness upon those who use it.
Having said all that, tone is not always an entirely irrelevant part of an argument and whipping out the "tone police" objection at the first sniff of a tone-based argument may sometimes be hasty. When making or observing an argument, you need to consider what the objectives of the argument are and the environment of the argument.
(By the way, I'm using 'argument' in it broadest sense, be it a fierce disagreement or a more friendly debate or discussion.)
The environment of the argument is often where the fuzzy edges of the internet (where most arguing appears to take place these days) can make things confusing. In the real world (or the wonderful term 'meatspace'. I love how we've started to describe the real world with secondary terminology, like 'snail mail'), it's much easier to pitch your tone accordingly. If you're sitting across a table from someone to whom you strongly object it is unlikely you would put yourself with in inches of their face and start screaming at them. At least, I hope you wouldn't - this is pretty abusive behaviour. You are far more likely to scream and shout if you're arguing passionately to an audience, raising a rabble or leading a march. It's not uncommon for things to get enflamed even in a one-on-one debate, because you are performing for an audience and not scaring the shit out of just one person.
This is where consideration of the objectives come in: what are you trying to achieve? Are you trying to change the mind of the one person to whom you disagree, or do you just want them to know how angry you are? Are you trying to convince an audience (be it a readership or physical spectatorship)? As sound and valid as your argument may be, it is naive to think you can be as effective in all situations with the same tone. It just isn't the case. While it is perfectly valid to shoot down tone policing from an opponent who wants to derail your argument, I don't believe it is as valid to shoot down an ally who wishes to strengthen the effectiveness of your argument.
I think we're too quick to do that.
This thought vomit sprang out from a discussion about Caitlin Moran over twitter. In Moran's case, she has shown that she is unresponsive to any form of criticism, aggressive or measured. In this case, what do you do? I think we have to accept she's not going to listen to those who think her dangerously narrow form of feminist philosophy is all kinds of wrong, so there are two contructive things we can do. The first is to deconstruct her bullshit for everyone else who may have read her work, or heard of it. This will expand the knowledge and understanding of your common audience and hopefully prevent or innoculate people from her bad rhetoric. The second is to let her know you disagree with her, and why (even if she'll ignore you). This will remind her that she keeps saying disagreeable things which may (optimistically) make her think a little harder in future. Firing abuse at her is not particularly useful or productive and does little more than ease the burning anger a little. There's being aggressive, and there's being a dick.