Jeff,
OO purists did loose you. Because to an “OO-purist” object oriented analysis and deign is about modeling reality not about some academic exercise. In the real world we have a phylum-order-genus relationship in everything, not just in the academic dog, cat, and animal scene.
The most fundamental discipline that it gives us is the art of abstraction. Looking at something in a domain and factoring it, concretely defining it and allowing it to be made more specific without loosing its core characteristics removes the error-prone, not-reusable tedium of working with the things themselves and instead lets us work with types of things.
But we would be remiss, Jeff, if to OOAD we ascribed only the virtue of reusability—for that is not just what it’s about. A much more immediate benefit can be found in generalization. Look no further than to the copy of (GoF) Design Patterns that must surely occupy a space on your desk—and in your heart.
Look at the Composite pattern. Based on inheritance and polymorphism, the pattern lets you perform operations on a hierarchy of differing node implementations using a single, simple, factored interface. I don’t know how many times I’ve found this pattern to be invaluable. It saves time during implementation, time during maintenance and time during addition of functionality because your system doesn’t need to know about each node. (I presume at this point you’ve inferred the meaning and the burden of “knowledge” as it exists in a type-safe environment such as .NET)
The reason inheritance and polymorphism work in the composite pattern and all of the other patterns in GoF, is because of the simple truth in my first paragraph: abstraction is crucial. If we can see the similarities in things then we can operate on just those similarities—without knowledge or even the desire of knowledge of what makes the thing unique.
When I have this amazing ability and lens with which to view my domain, what would I gain by doing things any other way? Job security, perhaps, when I have to re-factor my entire solution because I have to add something new to those simple, broadly-defined, objects I was using.
Inheritance, polymorphism and encapsulation are more than OO concepts; they are the three pillars–The Holy trinity upon which my church was built. We’ve all been exposed to the wide road. That path filled with the temptation and damnation of RAD, VB6, spaghetti code and procedural noise. But in the height of the corruption by the demonic forces of Redmon we were given pardon by the trinity taken flesh… the zen and the vision… light it up, Jeff… just one little hit off of the Booch-pipe and you’ll join my church.
(All of this is IMHO, of course)