Counterpoint: they require a lot of care and attention, and expense. You can't easily just pick up and go, you now need someone to look after the animal, every time.

You can get exercise and social contact besides (although on the latter point, animals can provide some comfort, but they don't sub for a social life). I think the valid reason for wanting a dog is wanting a dog, and tolerating if not enjoying the lifestyle that it requires. There are a lot of owners out there who think they want a pet, and then they end up neglected or at a shelter (or the owner does their job, but regrets their choice).