If you interact with your dog as much as you ought to you already share germs together, and if you have a dog I hope you are not allergic to it. Dogs strongly consider themselves to be part of your family, and they recognize comfort when they see it, so I see no reason they should not share a bed with you. People who do allow dogs in bed with them report the experience as a very happy one, mixed with complaints about how the dog hogs the bed offered with a strong sense of humor. I don't believe we should treat other creatures as somehow inferior, as if they don't deserve the same closeness and comfort of other (human) family members.
As long as you don't mean this in a figurative sense and mean it in a literal sense then I see no reason why not. It can be nice to cuddle up with a dog or a cat or a ferret or what have you. Just don't sleep with your dog in a figurative sense. Dogs have other dogs for that and don't need you for that.
Dogs can be such an integral and loving part of many families. They are affectionate and provide love, protection and assistance for many. It should be up to the individual owner if they choose to allow their dog to sleep with them, but I see no harm in that process. Certainly factors such as shedding, size, noise may prohibit that from being practical, but for those reasons only should it not be considered.
If your spouse came from the gym all sweaty and smelling would you allow your spouse to share the same bed as you or would you have them shower? Dog's don't bath themselves so if you let a dog out, you need to bath them before you have them climb in your bed. I for one don't have the time to bath a dog every single day.
It isn't like all members of a normal household share the same bed anyway. Maybe I would consider bunk beds to share with the dog, but I'd insist on getting the top bunk. Dogs are dirtier than people. They are happy to roll around on the ground, eat cat poo and drink their own vomit. To all you out there who let your dog sleep in the bed, you are begging to catch all manner of virulent diseases unless he is willing to bathe and brush his teeth at least as often as you do.
Humans are the pack leaders. If you allow your dog to sleep with you, you are telling the dog that you are all equal members of the pack and the dog gets confused as to who is the leader. Pack animals need a leader. If the dog ever develops a fear he needs a leader to look to or else he will take the matter into his own hands in a society run by HUMANS. This is unfair to the animal.