If you’re using something like a 6X or 7X tippet, the lighter rods have a more flexible tip to help protect those fine tippets. By matching fly size, line weight, and tippet size, one can better optimize the fish-fighting potential of all three to land fish faster and more conveniently and safely.
While there is truth in that, and I can tell by that you know what you’re doing, I sure see a lot of guys using 2 and 3 wts taking a long time fighting larger fish, when they should have a significant bend in their rod. When asked they invariably say they’re using a 3wt (or 2wt) and don’t want to break their rod. Those guys have no business with a rod like that. They do it at the expense of the fish. Why buy it? The rods can take it and are built to do it. I’m not down on 3wts but learn how to use them for crying out loud. About the only time I use 7x though is for Tricos and midges and sometimes use a 8’1” Kenney for that. But my glass 4 and 5 wts seem to protect 6x and occasional 7x just fine. But most of those can cast with just one foot of fly line out. Few graphite rods perform that stunt well and are not as protective of finer tippets. What I use my 8’1” Kenney for the most though is bluegills with 5x and they tend to find a way to jump into my freezer.