Do you care to unpack this? How is this a good example?
Just in case, maybe "good example" wasn't the best abbreviated term there. I meant it is a good example of bad coupling, where the better answer was to split them.
As to why, it's not open and shut, but in general I would say that you can judge it by the results. Race and class mixed is an abstraction that you can't readily scale. Taking strictly, it means that adding specifics is clunky. If you want to have elven fighters, dwarven clerics, and halfling wizards, you have to add those. And in the naming conventions, you don't even have good things to call them that fit the pattern. Furthermore, the combined abstraction implies that all demi-humans are alike while humans are varied. Finally, related to the previous point, the combined abstraction is inconsistent in the level at which it abstracts. It's supposed to be a combined abstraction of race and class, but it really tells us little about human society.
All of those points have valid counter-arguments. In Basic, they are pretty strong. Because Basic mainly doesn't care about any of that stuff, but does care very much about keeping the "pick something and get on with it" at the forefront. You might say something like, "Class is an abstraction. Race is an abstraction. We combine them, because in Basic we don't care about the distinctions. It's just a stock to get started with."
That brings up what I was saying about collapsing. It's often ok to collapse. It's not ok to lose sight of what has been collapsed when a designers changes the rules. A DM or player at a table using Basic can say, legitmately, "Hey, we are using this highly abstract race/class combination. If we want a dwarven cleric, we'll just use a cleric, file off a few serial numbers, have her sport a beard, and call it good. The important thing is to take the stock and run with it." Or any number of similar tweaks.
OTOH, if a designer takes that attitude (whether official or someone else attempting clean design), then it's off. Because now you aren't just supporting "dwarven cleric." You are supporting every combo. It should become obvious immediately that the list is going to get very unwieldly. In other words, said designer is now changing the assumptions that led to the collapse in the first place. So it needs to be split out again. Meanwhile, your coupling problems have compounded astronomically. Whereas "wizards" or "elves" getting to use item or spell X was fine, with only "wizards" using item or spell Y in Basic, now that we've got halfing wizards and human fighter/wizards and tomorrow gnome and goblin wizards. Turns out that "wizard" is a label that has taken on increased importance when linking outside the race and class abstractions. Your new abstractions needs to recognize that fact.
If not, what you get are a host of overly complicated rules about swapping this feature for that racial ability, or this spell for access to that item, etc. And these rules are not easy to modify. Total them all up. Then split race and class out again, and you find that you've eliminated the need for 80% to 90% of them.
Finally, note that this "proper" coupling is always a concept that comes with a great big implied "If": If you are going to support a bunch of races taking a bunch of classes, you need to split the race and class abstraction. If you are going to add a skill system, you need to think about what are broad skills that anyone could theoretically do versus what are class abilities retained in the classes themselves, and then abstract the class and skill boundaries appropriately. If you are going to have a lot more "magic" types than arcane and divine, how do you set up the boundaries, and what does this imply about the magic abstractions?
All of these decisions have costs. In Basic, the cost of the race/class split could be too high for what you get in return. In AD&D, the cost of not splitting is too high for what this would do to the complexity of the rule set. For any given piece of complexity, you've always got the options of abstract it, drop the feature, shove it off to the side (for only it's biggest fans to handle), accept it as necessary, or try to finesse it. The best course for any given piece is necessarily affected by the choices made for the pieces around it.
That is probably more unpacking than you wanted, but I'll let it stand now that I've bothered to say it.
