Even though I disagree, I respect your opinion.
Let me throw another scenario at you. Lets say I'm in the park with my 8 year old daughter, and a large dog approaches growling and snapping. Would you suggest the same course of action, or do you believe it appropriate to deploy the spray. Assume for the sake of argument that our dog is not with us.
Since I wouldn't carry pepper spray in the first place, it's sort of a moot question, and it makes little sense to answer hypotheticals about fictional dog body language like this. What dog on the attack would really approach as you describe, and how would you have any idea whether it meant a bite was imminent? Suffice it to say that I would risk injuring a strange dog if it meant protecting a child or my own dog. However, I don't regard subjective interpretations of dog body language to be cause for injuring a dog preemptively. A dog isn't definitively attacking until he has begun attacking.
There is a basic difference of worldview here. I don't believe that carrying a weapon is an effective means of protecting oneself from situations one is likely to encounter out and about while walking a dog. For each million-to-one hypothetical scenario in which you successfully stop a dog who was definitively going to hurt your own dog or child, there are many infinitely more likely scenarios in which you accidentally spray a harmless dog, accidentally spray your own dog or child, accidentally discharge the weapon in a harmless context and hurt a bystander or yourself, or never get it actually deployed in the first place. Seriously, how realistic is it that somebody could effectively and accurately deploy and discharge a weapon of any kind one-handed in the handful of seconds they'd have between making the judgment call about dog body language and the arrival of the strange dog?
And, at the end of the day, I've never seen a dog attack preceded by the kind of "growling and snapping" approach you're describing. Dogs who are going to attack, in my experience, come up slowly and stiffly with hard, steady looks, and it's a very nuanced judgment call to know if a dog is coming up nervously to greet or if he's coming up to make a true biting attack. Dogs that come up loud and mouthy usually turn out to be over-exuberant players.
It's just not a realistic scenario. Hypothetically? Yes, I'd have no problem risking injury to a strange dog if he legitimately threatened me, my dog, or a child I was responsible for. In reality? You can't walk around with your pepper spray—or whatever—drawn and aimed at any dog who approaches just in case he turns aggressive at some point.
I feel like my dogs and I are much more in danger from people armed with things they think will protect themselves and a false sense of their own judgment and accuracy. I can easily imagine my dogs being wasp sprayed by a dog-aversive person on a trail because they came within ten or fifteen feet before I called them back. I cannot, however, imagine how I would successfully get pepper spray into the eyes of an aggressive dog without an unacceptably high risk of hurting a totally harmless one—or other sorts of accidents.