
Honestly, I've not used Guard Dog a lot, but it seems to me that it's the Guardian ally for non-guardians. It can efficiently do 3 damage over time, for a reasonble cost, and it's quite nifty being able to invoke an attack of opportunity, soak it up and kill off an enemy.
However, let's face it, if you're a guardian, you probably just want to beat things - so Beat Cop is a more natural fit. And if you've got access to guardian level 1 cards, you could upgrade to Brother Xavier, who offers more, and still does 2 damage. He's better in every way.
So this leaves the place that Guard Dog shines as being with investigators with poor/moderate strength, and access to guardian level zero only. That's pretty much Jim, Rex, and Pete, and the latter two have better allies to take priority.
With Jim, though, Guard dog is pretty good, and it's health/sanity balance nicely compliments Jim's, and gives Jim a good way to do a little damage when he needs to.