Maybe they are just in a hurry? Maybe they are "too cool" to just follow the rules?
This procedure - I will opine - is idiotic at best and serves only a few solid purposes:
1. Piss off motorists who already have a disdain for cyclists.
2. Piss off cyclists who are trying their hardest to follow the rules of the road and not piss off more motorists.
Of course, the "hardcore," or those who think they are Lance, or those in too much of a hurry to wait in line like everyone else just don't seem to get it. They squeeeeeze past people just wanting to keep that momentum going. God forbid they have to brake a bit and switch a gear.
The primary reason this is BAD is as follows: it inevitably leads to a "right hook" where a threading cyclist has a car suddenly turn right cutting the cyclist off. This can happen any time for sure, but when a rider is threading on the right, a right turning driver is not expecting a sudden pass on their right by a "oh I cannot slow down because I am a wannabe" cyclist.
A secondary and worse thing that can happen due to threading through slow moving traffic in urban areas is what I will coin the "steel cage squeeze." This is even more ridiculous than the stopped traffic/right shoulder situation. You going to fly between to moving vehicles where you don't know if they are about to move closer together like the walls in an Indiana Jones movie? Darwin would be proud of you.
Motorists simply cannot know what the hell you are doing as a cyclist if you are riding all over the damn place like a hipster idiot. You are operating a vehicle - yes the bike is a vehicle - and by law should follow the same rules as other vehicles. Why? Predictability. That is a primary operational procedure on a bike that keeps you safe in traffic. Buzzing all over every lane on all sides of the road at three times the speed of traffic in an urban setting is asking for it.
I present to you this following evidence I came across via another site. Threading is BAD and DUMB. Threading can get you KILLED. BOTH of the cyclists in this video clearly play with fire (threading) and this my friends is how you get burned.
This guy needs to take at least one life off of his nine for this idiot move. I had to watch this a handful of times to just absorb it all. I know this goes on a lot - I see it in my own hometown - but this takes the cake. Lets do a quick play by play of these guys moves - maybe any childrens who happen upon this post will learn something - that riding a bike like you are in a Jackass movie will make you look like just that.
Cyclists: Don't thread. just slow the hell down and enjoy the ride. The WORK will still be there when you arrive. Your BOSS will still be a dick face wether you are five minutes early or five minutes late. The hip coffee place you are going to will still be there. Sheesh. Ask yourself: "Is it worth my life?"