It’s funny to me how people are either adamantly against over-lining (putting a 6- or 7-weight fly line on a 5-weight rod, for example) or totally in favor of it. I think I’m somewhere in the middle.
On the one hand, rods are built to handle specific lines for a reason. It seems to me, that the moment you put a 6-weight line on a 5-weight rod, that rod becomes a 6-weight, right? The advantages of doing this are that the heavier line loads the rod more, you can feel that when you cast, it can help you fling heavier flies, and bust casts into heavy headwinds. It can also help turn over a longer leader when you use one.
But some people worry that this is just a crutch. If you get too dependent on the heavy line, they say that ultimately fouls up your casting stroke. Why not just figure out how to cast an 8-weight rod with an 8-weight line in the wind, or else go up in size when you know you will cast larger flies?
Of course there are no rules for any of this, and I always admire how anglers make tweaks and adjustments to their gear to optimize performance. I’d never criticize anyone for over-lining, or refusing to do so, for that matter. I don’t think it’s like throwing deflated footballs in an NFL playoff game. But are you cheating yourself and your own skill development by using fly lines off label?
I think, done in moderation, in specific situations, it’s probably a good thing. But I’m interested in what you have to say.