There are several reasons why filtering to the front on a left turn is a very bad idea. First, if the left turning lane of cars has to wait on a left turn signal or if there is no left turn signal and the cars have to wait for on-coming traffic to clear, as a cyclist you are trapped to the right of the first left turning car and next to a line of cars that is moving on your right. People don't expect you to be in that position and, if someone is less than attentive, you could end up as a smear on the right front quarter panel of the first car in line.

Second, cars that are turning left aren't expecting to see a vehicle making the same left turn on their right. Again, you aren't in a place where a turning vehicle expect to see you. If the motorist makes a wider than normal left turn, you could still end up as a smear on the right front quarter panel of the first car in line.

Additionally, a turning car might not see you and could make a right turn into a driveway as soon as they turn left. This is a left/right hook combination.

By queuing up in line, you are in control of the cars behind you. You aren't sitting exposed to moving traffic on your right and, contrary to what some seem to think you are less likely to be hit by a car moving behind you. You are also predictable and the cars know how to deal with your movement. An on-coming motorist won't be confused by your actions as they recognize that you are turning left.