[pistoleer]

Flies: they get inside and nestle in my fruit, annoy me and distract me, get in my face.

Bees and wasps: they settle and build nests in nooks and crannies of roofs. I don't have a problem with bees per se, although they can probably keep disturbing eating in the garden. Otoh they may pollinate flowers in the garden. Wasps on the other hand are truly a pest. I've lived in a house with wasps in the roof, constant wasps in the attic, leading to an unusable attic for about a year.

Bats: no idea, never had them so far. But I've lived in a neighborhood where they were nestled inside the outer layers of roofs. Just like other animals I imagine they "shit and piss all over the place" so to speak. But they're also protected where I live, so once they are there, you can't even get rid of them.

[kjs3]

We have bats. IME, they're no big deal outside of the house (cute-ugly, but don't play with them), but if they annoy you putting up some bat houses away from where you don't want them seems to work. Inside your attic, otoh, they're a nightmare. If you suspect they're getting in, fix that ASAP. Most places (that I know of, ymmv) there are licensed pest management folks who can physically remove the bats.

[WalterBright]

Bats keep the mosquitos under control.

[kjs3]

I didn't feel the need to enumerate every advantage bats have. And where I leave, there are not nearly enough bats to keep the mosquitos anything like under control.

[WalterBright]

I had a huge problem with wasps. Wasps, everywhere, for years. Eventually, my cedar shake roof needed replacing. The roof contractor said is was full of wasps, as wasps like to nest in cedar shakes.

Replacing the roof with asphalt shingles solved that problem.