There are a lot of cons--more hair to clean up, more food to buy, more dog walking required, and higher expenses (your vet, grooming costs, etc.)
But, I always try to have at least two dogs. The pros are worth it. My dogs will play together for hours and hours, providing exercise for those days that I am in no mood to walk them. They eat together, sleep together, and generally keep each other occupied even when I am busy.
Single dog syndrome (only dog syndrome) occurs in some dogs that do not have another canine companion. They eat abnormally, do not seem to relate well to other dogs, and may have other behavioral problems like dog-dog aggression, seperation anxiety, etc. (Most of those problem dogs seen on the Dog Whisperer TV show were single dogs.) Some second dogs will learn things from another dog that they might never learn from a human: housebreaking, waiting at the door, responding quickly when called, and others.
And, of course, two dogs will give you twice the affection. What could be better than that?