Birds that come to your yard can pilfer nuts from trees and eat your fruits and vegetables, and a bird bath may also attract unwelcome animals like squirrels and rats.
The all-around messiness from a bird bath can muck up a nice patio, and having a large bowl of stagnant water can lead to issues that are unsightly and even dangerous.
Mosquitoes that carry serious diseases like West Nile Virus are drawn to standing water in bird baths, and the stagnant water can also be a breeding ground for avian diseases.
A bird bath needs to be cleaned regularly, which can be hard for busy folks. If you're fine with the upkeep and don't mind the downfalls, though, you'll likely enjoy having one.