Downsides?
Noise complaints from neighbors with possible intervention from AC.
A dog that barks excessively is likely a sign of something not right - boredom, needs not being met, anxiety, etc. Although there are some dogs that simply enjoy talking.
As for watchdogs, a good watch dog will only bark when there's a real issue. For a dog to bark at anything and everything doesn't serve well. A good watchdog should know when to say "Hey there's someone here!" and when not to. I don't need to be alerted to the neighbors across the street, a squirrel running through the yard, blowing wind. That much alerting is likely going to get ignored as an alert. A dog who is usually quiet and then alerts when a stranger is approaching his house is much more effective.