To me, saturation defines the line between proper and improper usage. For instance, I've used stock photos that also ended up on websites of competitors, and other organizations that would put our brand in a seriously negative light, or at the least, dim it. I'm in favor of using stock where appropriate, but I'll err on the side of not using it unless necessary, and do my best to use something people haven't seen.
The same goes for fonts. Some of them (*cough* papyrus *cough*) are simply used way too much and devalue the overall effectiveness of a once unique element. The average person may not care, but as a professional, I do.
I think you're right, Stock is just another tool, and in the same vein it's usage should be balanced with genuine creativity. There's no other way.