In the case of the Trent I think you will find that the heavier rods are used more for chucking out heavy gear rather than in consideration of the battle with the fish itself.
Not always, but often.
I've recently gone down in line strength when float fishing from 8lb main and 6lb bottom to 6lb main and 5.6lb bottom but there are swims where I wouldn't trot the lighter set up anymore.
On the heavy Avon rod and lead I generally use 10lb and the same for the bottom or occasionally 12 & 12 if it's snaggy, that's with a rod I rate at 1.6lb and would consider is a barbel stopper and there's one member on here who witnessed just that when I was fishing close (within inches!) to some tackle chomping structure before landing a 14-14 late summer fish.
Had that been 6lb line you could forget it but it also demonstrates that rods which don't necessarily conform to the usual (?) 5:1 maximum line rating to test curve ratio are more than capable of stopping a lump whereas other, heavier rods just might not have had they bottomed out and locked up which is easily achievable even with a much heavier rod.
I think it was Sam who said it earlier, it's horses for course and I would add that there are too many variables to advocate one thing over another as being standard recommended practice and those variables extend to the properties of the gear as well as the swim and river characteristics... Just my opinion mind, I state nothing as fact.
Graham...
If you're wondering what I'm doing back I forgot me hooks
mg: