Twopeasonepod - I think we kind of agree re. BC's. They are working dogs that need a job and they are not really suitable for the average pet home because of their needs. I'm sorry to hear about your grandas dog, but I would bet there was something else going on rather than it just "being nasty". I'm not insinuating that the dog wasn't looked after properly, or not cared for, in any way, but I've dealt with 2 dogs like that - they were PTS for a similar reason (biting) and one turned out to have a brain tumour and the other one had other issues that added to the situation.
In the same way, JRT's and other small dogs are treated as "mini people" or "pocket pets" and not allowed to be dogs... and don't even get me started on handbag chihuahuas, or other "cute" dogs that are a fashion accessory and so they are often snappy, yappy and generally horrible. I dread seeing one particular dog walker in the park if I'm there, because she has the yappy, nippy, out of control JRT's from hell, who attack everything fucking things. That's not their fault, it's HER fault for not keeping them under control.
Regarding an "instant command to leave" - I would be the first to say that it's not 100%. NO dog is 100% reliable. But, I also know that my dogs, having been trained with the command, over and over and over, are a damn sight more likely to leave something if I tell them to, than not.
You are perfectly entitled to be wary of Border collies, in the same way that I am wary of huskies having been bitten by one, but I doubt that you scream "SPAWN OF SATAN", point and get hysterical when you see one and tell people what evil dogs they are! You just treat them with caution and respect.