I absolutely agree, with the exception that in many of these cases time is of the utmost importance. If someone was generally asking about a good test kit, I would always (and have always) recommend shopping around online and finding a good deal. However, when there is a mysterious situation and there is the possibility that fish are currently suffering from acute ammo / no2 poisoning, I'd happily spend an extra $20 to get to the bottom of the situation as quickly as possible and make sure I'm in the best situation to deal with it. I view it the same way as if one of my dogs seems to be in distress, I don't wait until the regular Vet opens, I climb into the car and head towards the emergency vet clinic knowing it'll cost me hundreds of more dollars than waiting until my normal vet office is open.
I completely agree with your statement, but there are always exceptions to the rules.