As mentioned before, I'm from the UK, have always done it and in fact, was trained to. I even needed to do it, in order to get to the test centre in time for my motorcycle test, due to a traffic jam we encountered on the way there! To me, it seems hilarious that any motorcyclist of any sort of reasonable level of competence would be against legally being able to choose to do it, when conditions permit doing it safely.
This video pretty much demonstrates my own attitude/approach to filtering, when in a large city like London, where filtering is commonplace.
I was riding there, not ATGATT (as I was only riding 2 miles), in below freezing conditions earlier this week. I was still filtering, albeit more cautiously than normal.
To those who don't do it and think it shouldn't be legal: I think the single greatest factor is your fear based on your perception (put bluntly; your uninformed opinion) of the risk associated with doing it.
I like my bike because I can overtake 4x4s down farm tracks with a week's worth of shopping on the back.