There are many classes that could be usefull for configuring custom binding element and custom bindings. But their constructors are madeinternalplus there are plenty of usefull methods that configure binding elements that are also madeinternal.
Woudn't it be better if I could reuse HttpTransportSecurity security in my classes for storing the custom binding configuration reusing it when I need to configure the HttpsTransportBindingElement instead of inventing my own proprietary configuration classes?

Same applies to many other usefull classses and methods that are presently visible only using reflection. At the end WCF framework is very flexible but obstacles like this minimize it flexibility, giving only two options really either to use predefined binding classes or to implement everything from scratch and use CustomBinding. As I say there is plenty of code that could be used and save lot of developer time if it would become open.