For what it's worth, the old school sliding scales didn't have carb counts because the carb counting was figured into the diet—back then, I was supposed to eat a set amount of carbs at each meal and snack (1 diabetic "bread" exchange = 15 grams of carbs). Sliding scales should work in theory if you do that; the problem is few of us can or want to do the exact same thing every day, so having a more flexible system that allows for both variability in a correction factor (similar to the old sliding scales) and a carb ratio (the new aspect) is much more realistic for most people, including myself.
That said, correction factors and sliding scales both tend to make the error of assuming a linear relationship between blood sugar and correction doses. I know for me, once I get pretty high, the curve gets significantly steeper, and I start have to tacking on extra units. I think that's fairly common.