Why should I spend an extra hour writing a bunch of classes chock full of properties, methods, and constructors when a simple class with a bunch of "functions" would work just fine?This is a very valid question. The underlying reason many people even ask this question is because they are thinking small. They don't realize that the functionality or effort that they are placing in their utility could be utilized by someone else for another purpose. This all boils down to coupling. Coupling is the term that expresses how dependent a given module is on another module.
For example, I have noticed that many PHP applications are written in a tightly coupled fashion. In a given method or function, the developer is performing data validation and sanitation, manipulating inputs and performing calculations, and finally is executing data backend transactions. That is highly coupled code. If you wanted to reuse that data cleansing and validation functionality, your only option would be to copy it into your own method. Ok, thats no big deal. Now imagine that the backend database changed and the cleansing requirements change. Now the nightmares begin. In how many different areas was this cleansing code implemented. Worst off, how many different times was it written. Developer B did not like Developer A's variable names so completely renamed all of the variables and changed some of the logic design. In a loosely coupled design, the data validation and cleansing functionality would be abstracted into its own set of classes under some Data Utility namespace. Now requirement changes are a simple line edits in a single location away.
Now rethinking the previous example. Suppose the developer had written the application in a loosely coupled fashion however all this code is stored under the developers local hard drive. How does this help the situation. All code that is written should be stored in a central versioning repository. In addition, an entry should be made into a Wiki or other central publishing store so that other developers can find this code easily by category, namespace, or search. Personally, I really like the MSDN style of documentation. In fact, there are many utilities that can automatically generate this documentation so all you need to do is write up a small blurb about the class, method, etc and some skeleton code as how it should be used.
So what are the disadvantages of designing applications with loose coupling in mind. Well one is performance. I will not try to disagree with the procedural type developers and argue that loosely coupled code is faster. With proper design techniques and resource management you can minimize performance issues. Despite this, the cost of hardware these days is small but the cost of labor is quite high. In addition, having loosely coupled code also allows you to be a lot more nimble as design changes can be easily implemented.
Another difficulty encountered with loosely coupled code is that of deployment. Versioning of all levels of components is CRITICAL! A proper versioning pattern should be in place to ensure that every release (whether beta, alpha, production, release, internal, etc) is properly versioned and its features, bugs, known issues, and API specification is well known. In addition, with loosely coupled code, you want to make sure that your resulting libraries maintain API compatibility through the major version number. That means that if you have a typical versioning scheme of Major.Minor.Revision.Build, a dll with version 126.96.36.199 should be binary compatible with 188.8.131.52. That means, you should be able to reference any 1.x.x.x dll in your application and it should work (minus any bug fixes) without changing a single line of code.
One addition critical element required for loosely coupled design is refactoring. Refactor is the name given to the process of changing the code design without changing the functionality or results of the code. I love the word balance and try to implement it always in my life. I also keep in mind balance when designing applications. Sometimes, I will tolerate a certain degree of coupling because the costs of trying to abstract the code further outweigh the benefits. Since we live in a dynamic world, sometimes the benefits will start to outweigh the costs. As soon as this scale starts to tip, its EXTREMELY important to refactor. Many companies don't encourage refactoring because it does not make them any money. Your not fixing bugs and not introducing new functionality. Despite this, the long term cost advantage is hard to deny.
So whether your a small single developer or a large enterprise, loosely couple code along with proper versioning and refactoring can be a way to dramatically reduce development time and cut costs while increasing quality and delivering greater value.
Yes it is possible to have your cake and eat it too.