Any change to the biodiversity on our planet will have a negative effect. What is a pest to you is food for another species, or a pollinator, or any of dozens of valuable purposes.
They're not just a pest to all humanity, but dogs and other animals as well because they can carry a parasite known as heart worm.
I'm sure there are a bunch of other terrible diseases they carry as well.
Bats aren't solely dependent on mosquitoes either. For all I car they can find something else to eat.
When you've seen family members you love suffer from dengue, malaria or whatever other fked up shit they spread, it becomes personal.