MK, I see your point being based on "factory rod weight rating". Most rods will handle a range of 3 -4 line weights. A lot of the newer fast action rods on the market today seem to be rated towards the bottom of the rod's line weight range. Hence a 6 wt rod will easily handle 6 , 7 and 8 wt lines but the best distance will still come from the lightest thinnest line the rod can carry (in this case that would be a 6 wt.)
The idea behind "underlining" assumes the rod is rated at close to the mid range of what it is designed to carry,,, but the same principle that the best distance is achieved with the lighest thinnest line still applies.
The debate then (if there is one is) centers on the rodmaker's line weight rating of flyrods.
Grinr, perhaps a better way of describing how line weight choice affects rod action would be to say changing the line weight changes the "rod feel". Having 50 feet of heavier flyline out will cause to the rod to bend further down the blank giving the rod a "midflex feel". 50 feet of lighter flyline will put the bend closer to the tip giving the rod more of a faster action tip casting feel.
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