I write about design, and it's my view that even when you are discussing somewhat abstract design patterns it's useful to provide source code examples. Of course this can lead to people thinking that the code example is the pattern, but I think that risk is outweighed by the precision that code provides. Several times I'm not quite sure about an idea but a code example helps to clarify it for me. So in my writing on design I always try to provide code examples.
There are several ways of doing code examples. Many readers like complete examples that show how multiple ideas interconnect. I take a different route. I prefer very small focused examples that show only one idea at a time.
The problem with realistic examples that show multiple concepts is that they are hard to understand because you have understand all the concepts to get the example. With a focused example you can concentrate on one thing only. However with a focused example you don't see how the concepts fit together. Ideally you should provide both focused examples for the pieces and interconnected examples to show how they fit together. I confess I don't have the energy to do both, so I only show the focused examples. I reason that people have an easier job of fitting together examples once they have the basic patterns down. Also other authors can build on my stuff and provide interconnected examples. (Interconnected examples are a step harder due to the fact that I like show alternative patterns, so that leads to more interconnection combinations to show in a larger example.)
One of the tricks of the focused examples is to keep attention on the point of the example. So one of the things I do is to keep everything else simple and out of the way, which means I avoid using other patterns if they are likely to cloud understanding of the core issue, even if you'd use those patterns in a real system. An example of this is in the object-relational mapping patterns in P of EAA. I show a lot of mapping patterns (such as Foreign Key Mapping) where I hard code the data transfer between objects and database. But in many realistic systems (and certainly in OR tools) you would not hard-code those transfers, instead you would use Metadata Mapping. I show them as hard coded transfers because I think that makes it easier to understand what's going on, and also allows you to understand Foreign Key Mapping without having to understand Metadata Mapping.
Another consequence of keeping things simple is ignoring edge cases which ought to have error handling in real code. I feel slightly awkward about this, there's a tension here between keeping example code simple and ensuring that example code shows good habits.
This relates to why I don't provide code samples for download. I avoid downloads because the code examples I use contain lots of string and duct tape in the areas outside the pattern. Such things as generating ids for inserts from a static field - which you must never do in a real system. I do it because it easier for my simple examples, and I don't show people the scaffolding in the book.
It's also true that downloading the code misses the point. The point of the code examples is the ideas that the code expresses not the code itself. You have to read it to understand, you can't just throw it into an application of your own. One advantage of a code download is that you can step through it to help understand how it works. That's a valid point, but the duct tape rather spoils this effect. I think downloaded examples work better for an interconnected example than for focused examples.