Haha that is quite spot on indeed - I guess it's over-diplomatic polite talking and it works fine if you are talking to someone using the same system, but it can be a pain in the foot if you aren't as like the table shows, people can take it quite seriously and matter-of-fact, which can mean many unwanted obligations...
I find i have the opposite (almost) of what you say, i find myself being over diplomatic on forums and online, and while i often speak like this in real life it's more often than not with regards to things that aren't overly interesting. But in any debate or discussion that is interesting i tend too talk quite bluntly.
I think it depends on where you go and who you talk with though, my guess is this is most pronounced in the middle-classes of the south. It's a bit like the whole apologising thing, i think it falls into the same category. If you have read Bill Bryson's 'Notes from a small Island' you will know what i mean, but it is common practice for the victim of say a minor mishap to apologise as well as the perpetrator. For example if someone knocks into you on their way past or something similar, it is most common to hear the person who has been knocked into apologise for daring to be in the way of the other person, followed closely by an apology of the person who knocked into them.
Quite funny and interesting!
Kind Regards,
Sam Jackson