By their nature, lightline rods generally tend to be less stiff, therefore will allow you use lighter leader with NO worries
The problems arise with people using very stiff, heavier rods often 7 and 8wts on a water where they need to fish size 14-16 flies, then the shock absorbing properties of the rod wont allow for sudden impacts to light leader material.
I rarely use anything more than 4lb leader (often less) and catch many fish well into the mid teens, I am using smooth progressive rods with nice limber tips, usually 3wts often 4 or 5wts, never higher.
Unfortunatley you can't create a rule that specifies what type of rod to use or even , if you are using an 8wt then you must use at least 6lb line, it would be impossible to police. So i guess to allow for this , this kind of rule is perhaps an idea,The more enlightnened anglers are not affected by this until it becomes a rule so perhaps you just have to adjust how/where you fish
The Main issue as regards the line weight though, is surely what material? Many flouro/co-poly's are more brittle than nylon so 6lb co-poly although thinner than nylon wont stretch as much so with the stiff rods the nylon will break less often, the co-poly will still snap. Down side is you may get less takes on the nylon. 4lb nylon will usually break after 6 lb co-poly on a stiffer rod, the modern materials are more suited to progressive curve rods (which most of us use these days anyway).
Its a minefield issue, with a lot of parameters and still impossible to police,
If i have a spool of line with 6lb written on it and have it spooled with 4lb how would you know
Do you test evryones tippet? Bearing in mind line degrades with time and use! I dont think so. You will rely on people being honest. Or perhaps you could only inspect people who seem to catch more fish
I dont know where this ends, just ask Mike about how to police, barbless hooks
Sandy