This is just a guess but I assume that the longer and enemy (like a praetorian) is affected by the pheromones, the more enemies it might attract to be frozen. Increasing the effectiveness of the combo.

I played an almost identical build the other day but with extra cryo bolts instead. It felt like I had way more than enough cryo bolts but more pheromones would have been nice. The M1000 and pheromone bolts are your main source of crowd control and you only need 1-3 cryo bolts to freeze any given bug(s) so his concept makes sense.

Completely true, though everything fire arrows can do, cryo just does better and more