I agree that the one piece rod isn't about improving accuracy, but doing a lot of cleaning and shooting is. I don't think the one piece rods are necessary unless you are going to shoot and clean a lot. You theoretically could cause some damage to a steal barrel with an aluminum or brass rod (and decrease accuracy), but I can't imagine that happening if you're paying attention to what you're doing? People used universal rods for a really long time with few problems.
Now if you are going to shoot and clean a lot in pursuit of accuracy like bench rest shooters (who are the ones that came up with one piece rods) then its worth the extra money for a good one piece. A good one piece isn't going to flex and wear on the barrel like a universal, but it takes a whole lot of cleaning to actually put wear on a barrel with a universal rod. Most people aren't even going to come close to cleaning enough to cause that kind of wear. Cleaning just once a year it would take several generations for that to happen.