I'm guessing the dogs would also be able to sniff out ores, and you could probably (In complete Fallout 3 fashion) tell them to go get you food or other small items either from your house or from the wilderness. This could be exceptionally useful, IMO. And I'm guessing you would need to attach some kind of item to the dogs to make them no longer be wild, (similar to the pig saddle, but with an actual use).
So it wouldnt be reqiuired to have a dog companion.
Then again, I am a major fan of Fable and Fallout, so dog companions make me happy no matter their defects.
I like the dog idea, although it will be annoying if they get in my way or push me around. But having a companion will be enjoyable. Especially so if the your dog defends you or fetches requested items.
dogs in minecraft.. how will that work? lock it in your house while you mine and come back to your furniture torn up, wood floors scratched then find it sleeping in your bed? I dont really see what theyd be good for especially if they weren't persistent, unless theyre another neutral mob that could turn on you if you hit it.
About a backpack.. you can already carry 2300 bookshelves in your pockets, how much more would you like to carry around? A backpack should allow you to carry around a buick lol.
IMO: The dogs would be a nice change to wandering around in mines by myself. Having a creeper blow up your dog rather than you would be much better. Also, I've heard that they can sniff out gold? is this true? Why would we need a backpack? We already have a ridiculously large RPG pocket that can fit over half a 100x100 castle inside..