It has been common practice in traffic signal design that left-turn arrow lenses are installed (in Alberta they are called 4-section heads) wherever there is a dedicated left-turn lane and in anticipation of possible future activation. As a result, some may be installed at intersections in the City where they are not currently warranted.
We regularly review the need for left-turn signals. However, we must balance the need for left-turn arrows and the needs of the oncoming through traffic. Each time we activate a left-turn arrow, there will be negative consequences to the oncoming through traffic. Typical negative impacts to thru traffic on major corridors are a lack of signal coordination as well as 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 benefit side street traffic at the expense of main-street traffic having to stop. When we have signal lights at many consecutive intersection, such as along Mayor Magrath Drive South, signal coordination becomes 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 oncoming through traffic having to stop.
Again, this is particularly true when dealing with major corridors such as Major Magrath Drive, Scenic Drive, 13 Street, 6 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, when the pros outweigh the cons, they will be warranted, and traffic engineering will be completed to minimize any negative consequences.
A web search will turn up many online FAQs that provide detailed explanations about left-turn signal issues.