As I mention first, I, myself am a pet owner of a dog. My choice is to leave my dog at home. Dogs can be calm but not every second. Dogs crave attention and love, which you give to them after work when you aren't busy. When you are at work you must focus and concentrate on your work. Dogs can be time consuming, you would pet or play with your dog during a break of five minutes but a dog needs more attention than that. It gives more opportunity for procrastination and not getting work done.
Secondly, I admit dogs can relieve stress and provide comfort to owners but at what cost are willing to take to have your dog around? You could get side-tracked by taking the dog out or playing in the office with it when other people are trying to focus on their work. Dogs are distracting, when a person who isnt a dog owner is trying to work and gets distracted by someone else's dog they aren't getting any work done.
In addition, yes, I am reluctant to leave my dog all alone. We have to think of others, some people are allergic to dogs and maybe even some people simply just don't like them. No one needs to have unnessecary allergies in the office. Instead of perventing sick days we would be creating even more of them. Dog owners can consider a friend watch the dog or hire a dog sitter.
Lastly, dogs should be kept to at home. Not at a work place, there are too many complications and not enough solutions for dogs to be in an office. Bring your dog to work program is not nessecary and should be reconsidered in consideration of not just people with dog allergies but for the whole company itself.