choose right tool

this is one of my favorite 3 wt rod, yellow fenwick fiberglass rod from Japan. I matched it with 7 weight line in this skinning water fishing situation. What? 7 weight? That's right... 7 weight line on 3 weight rod to fish this skinning water. The key word here is "skinning water". I only need a 10 feet cast to deliver my fly in this situation. so the 7 weight line would be my choice on this soft rod. And it will load the rod properly for a 10 feet cast (with authority).
On the other hand, if I match this rod with a standard 3 weight line, it wouldn't load the rod properly that I have only 10 feet line out, because the line weight was calculated by use a standard 30 feet fly line in the front. This is just one of the many cases to demonstrate that rule is just for reference, but to follow completely...