Jeremy's course does a good job of explaining that.

>To the degree they're not used enough, it's (partly) because the developer isn't spotting the pattern where interfaces are necessary. And even when the developer does, it's a matter of knowing how interfaces can help.

That is an issue
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>It's the old "chicken and egg" issue: it's hard for developers to really "get" patterns and anti-patterns until they've written some code and have seen inefficiencies. But on the other hand (you can see where I'm going with this)
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WCF. Bleh! The first task at my last job was pulling out what we called WTF. Again, a case of the developer's not knowing when and how to use it properly. It was way overkill. They had gone to a WCF course taught by a well known author who recommended WCF everywhere. They bought into it.
>I didn't initially spot the need until I got into .NET remoting and until I tried to write my own data framework. Then it became obvious. Kevin McNeish's book on moving from VFP to .NET covers the fundamentals very well.
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>Another good way to show someone the value of interfaces is to show how someone would do a distributed app with WCF, and then without WCF. Even with the challenges of WCF, it's still a good way to introduce interfaces.