Does it mean, you think if there is already some mechanism, then it should consume the same amount of time processing 20 units as processing 10?Nearly all the mechanisms are in place. It would introduce more animals onto the board and there'd be more units operating like criminal units currently do. Would it really mean THAT much more processing?
Criminals wander into cities spreading crime. Pests would be basically the same thing spreading instead disease (as a property.)
The first page says, we have already 15 pests. Let's say there is averagely 7 pests per city. In an advanced game we can probably have 100 cities, or more. This gives already >700 additional units to process. And we are not counting traveling pests and pests' predators.
Not all great ideas can or will be implemented, but you still have to keep coming up with them. Because without them, you're left with either doing nothing, or doing things that aren't worth doing! 

You deserved a cookie.
and -1
. Should a rat catcher cancel out one of the effects? Likewise we already have the Rats get +1 
to cancel out the +1
, +1

with Flies. 