I only split lanes on one section of my commute that for some reason is totally gridlocked about half the time I ride home from work. The other half of the time its 2 lanes of not much. Bad traffic planning I guess. Anyways, I filter there because theres never enough room at the curb (every fifth car will be jammed right up against it for some reason), and because theres less chance of getting doored.
Every once and a while some guy will be blocking my path because he has done a half-lane change, so I have to do a really tight U between car bumpers towards the curb and then back into between lanes. I also watch out for pedestrians and found that even when close and audible a bell is useless and you MUST yell at them.
There is no way in hell that I will EVER wait in line behind 2 kilometres of cars for an hour sucking fumes waiting to get by a section I can ride in minutes.