"The desire for perfect health" is a great example Godfrey. Perfect health is unlikely for most of us, especially after s certain age. But we still want health "as close to perfect as possible."
What I am wanting to see here is a good explanation of how we can recognize that while an extreme of a good may be unobtainable, a lesser amount of that good is definitely desirable and working hard to get, and any formula we develop has to take that into account.
This is why I still am not sure I see anything in the natural and necessary categories beyond what Torquatus pointed out as the reason for it, which is a good reason (as a tool of analysis). But this is a tool that in the wrong hands is being used to bludgeon the philosophy of pleasure into submission in the modern world. At this point I am still at the point of thinking that in today's discussions (the world at large I mean) we are finding ourselves devoting more time to containing the damage it does in the wrong hands to the benefit it provides us in aiding our analysis.
Whoever Cicero got that Torquatus material from seems to me to have been pretty sharp. As he says, only an idiot (my overlay) pursues pain for the sake of pain, and people need to smarten up and examine their actions so as to predict whether a course of action will produce more pleasure or pain. The more extravagant the goal, the more likely it is that extra pain will be involved. And considering whether a goal is "natural" and "necessary" helps us predict the amount of pain likely to be encountered.
Now no doubt we can use the categories to give all sorts of examples, but in the end the examples get very particular and specific very fast, so the general rule remains something like "be prudent in the way you pursue pleasure."