Toronto is relatively balmy compared with every other significant Canadian city east of the rockies and it's not in the snow belt.
What it does get is vast seas of road snot a pedestrian has to wade through at every intersection. That alone is reason to stick to PATH between October and May.
Expanding on the other comment, it's the mixture of snow, slush, salt, and loads of sand/grit that gets churned up by car tires into a brown and sticky slime. It sticks to boots, gets caked on cars behind the wheels, and gets tracked shockingly far into buildings. It's sort of like a "slushee" consistency but made out of nasty brown goo instead of corn syrup.
What it does get is vast seas of road snot a pedestrian has to wade through at every intersection. That alone is reason to stick to PATH between October and May.