Making full use of the road to maximise observation...is what its all about.
The issue is judging speed of oncoming vehicles, its not for beginners. Get it wrong and there is a very nasty accident potential. Get it right and the trade off is potential for swift progress through traffic queues etc. Its like filtering to some extent , as both are risky. Filtering much more so, but the issue with off siding is just at what point is it not risky...an advanced rider can justify their own techniques, but trying to pass that sixth sense, that reaction time etc to a pupil is impossible, so IMO its not for beginners.
The iAM will expect making use of your lane though , but always with caution regarding on coming traffic.
Plus people like to show off a bit , show how " advanced" they are etc to others,might be tempted to push the safety limit a bit ,let alone the issue of riders following their leader ! so in todays litigacious society I aint surprised its being dropped. I passed my IAM 14 yrs ago, and riding techniques were much more towards police standards back then, you were expected to show intelligent overtakes by off and near siding , making full use of bike acceleration , but showing restraint for all speed limits. Its actually quite difficult to get all that to meld!
Also dont forget a cop bike in full highviz will also get some respect from oncoming traffic, which we just wont ! It has to be a safe manoeuvre.