Just trying to understand why limit the options if a balance control serves the same purpose.
Having Gain Trim controls allows you to not only set the balance but also the overall gain of the line section. An example might be if you have a digital source which typically has an output that is quite high (too high IMO, but that's a different matter).
With a balance control, you're pretty well stuck with the gain of the line stage which might mean that your Master volume control is most of the way down. This can make it difficult to use. With independent gain trim controls, you can set the overall gain structure and use the Master volume control as you see fit.
