Umm why worry about house cats fighting humanoids? why have battle field stats for house cats ...
It's a reasonable question, and you can try to answer it in alot of ways.
One way to answer it is the campaign that I'll probably run after I finish the one I just started (say in 2-3 years) has also been floating in my head for years, and the second encounter of it involves a 'crazy cat woman' and a house full of psychopathic cats. Under those circumstances, the fighting ability of a cat is important.
Another way to answer the question is that the 'house cat problem' is short hand for a number of issues that arrise in most game systems that have to do with the ability of the system to handle scaling up and down, and a particular issue relating to D&D's use of HD to track the ability to sustain damage. The 'house cat' is the most easily communicated example, but the problem is actually far more pervasive than that and in my opinion includes things like:
a) Many large creatures need artificially inflated HD. This for example results in whales being very high HD creatures, which, since everything is tied to HD, creates possibly unwanted side effects. Other creatures have ad hoc hit point bonuses. These are symptomatic in my opinion of failures to consider scale very often in the rules, particularly in pre-3e versions of the rules.
b) Arguably tangetal, but I think at least related to the cure is that Elfs are about the only creatures that have traditionally had below average constitution. I believe this is because too little emphasis has been placed on scale, and instead constitution is often treated as a relative rather than absolute value. For example, rats are 'healthy' so despite weighing 10 pounds, they get attributes implying that they medium sized and ad hoced 1/8th HD. But this implies that rats and 200 lb humans need the same dose of poison to kill them, and that a snake bite is equally lethal to the two.
c) Until 3e, being small carried no concrete drawbacks at all and being large typically was a pure penalty. In 3e, some attempt was made to deal with this and scaling in general, but the system still largely makes being medium sized an effectual penalty and being fine sized is in and of itself a huge benefit. This is potentially quite undesirable, as it suggests that not only is it likely that a housecat kill a human, but a wasp or spider has a decent chance of it as well.
d) One hit point of damage actually turns out to be a fairly large amount, but D&D has no way to scale down lower than 1 hit point of damage. This problem of not being able to scale down is actually shared by most systems.
And lastly, I'd agrue that if possible the rules should be such that a novice DM doesn't need to know about all the hidden gotchas in the rules in order to know when to not apply the rules as written. If not, why try to have versimiltude at all?
At some level I do agree that the system can be run while succesfully ignoring the issue, but I also think that the problem is not so difficult that it shouldn't be solved. If it was difficult to solve, then maybe it wouldn't be worth it. But I think I think I have largely solved it, and the solution is fairly straight forward.