I'm an electrical engineer with some amateur radio experience as well. Everything said here is right MOST of the time. If all of the components in the receiver are tuned to their optimum values then you'll be fine. However, electronic components are generally specified to be within a certain tolerance of their listed values. Some are at the middle of the range, some at the bottom, and some at the top. If you get enough components that are at the wrong end of the tolerance scale then you may have a reduction in receiver performance that is noticeable between high band and low band.
My guess (and that's all it is) is that the Futaba engineers are trying to make sure that their receivers are ALWAYS in spec even if the components are skewed one way or the other. Since they charge the same price for high/low band receivers and it would only be an issue if you hopped 30 channels, I doubt it's a marketing ploy, just cautious engineering.
Range checks only tell part of the story. Unless you have a way to measure things like signal to noise ratios you won't know how close to failure you really are until it's too late. Either way, it's your plane.
