It has been common practice in traffic signal design that left turn arrow lens are installed (in Alberta they are called it 4-section heads) wherever there is a dedicated left turn lane and in anticipation of possible future activation. They may have been installed at many intersections in the City but not because they are currently warranted.
We constantly review the need for left turn signals. However, we must balance the need for left turn arrows and the need for the opposing through traffic. Each time we activate a left turn arrow, there will be negative consequence to the opposing through traffic. Typical negative impacts to thru traffic on major corridors is a lack of signal coordination, and stop and go movements.
It may be simpler to compare it to new traffic signals. Each time a new traffic signal is installed and activated, it will be easier to the side street traffic at the expense of main street traffic having to stop. When we have many signal lights, such as Mayor Magrath Drive, signal coordination will become extremely challenging. The same can be said for left turn arrows: each time a left turn arrow is on, left turners get to go, at the price of opposing through traffic having to stop.
Again, this is particularly true when we are dealing with major corridors such as Major Magrath Drive, Scenic Drive, 13th Street, 6th Avenue South and Stafford Drive - where signal coordination/traffic progression is desired.
Our traffic group monitors traffic movements for left turn warrants frequently. Simply put, once when the pros outweigh cons, they will be warranted, and traffic engineering will be completed to minimize any negative consequences.
There are many website FAQs giving detailed explanations about left turn signal issues. Here are two links.