Most examples of open classes are extensions of core system classes (string/float), such as adding a "to_cents" method on Float. Often such extensions are not needed, as a new wrapper class, such as "Money", would serve the purpose better.
Core classes may not need to be open for extensions, although that can sometimes be useful (remember how long it took Sun to add java.lang.String#replace(String,String)). It is important for Model classes to be open for downstream extensions. For this reason I use the term "Open Concepts" to indicate that the concepts in the model are extendable.
The biggest advantage for having "Open Concepts" is that the model can be modified without modifying the original source. This creates many opportunities for multiple team system development. By enabling the model to permit dynamic integration of independent and decentralized designs (open), the code becomes simplified and more useful to a wide array of use-cases. Consider the productivity advantages of allowing downstream development to have more influence to extend the supplied model without necessarily involving upstream in the changes. These are many of the advantages that allowed the Ruby language to gain so much popularity.
There are many ways to enable this, even in more mainstream languages. One popular approach is to implement a plug-in system for key points in the model (although this quickly become complicated with many "key points").
A more generic way is to implement the role class model in all the concepts. This allows every individual to expose itself through multiple concepts. However, anyone that has used Mozilla's XPCOM, knows first hand how the syntax of this pattern can get out of hand.
Today, with the popularity of new languages that support the run-time mixing of classes, new opportunities for creating an expendable model now exist. Such an approach allows both plugin-style and role-style extensions, but still uses the syntax of the host language. This allows the model to be used the same way as if no extensions are present.
Any large system will need some type of extendability be it a plugin-style, a role-style, or an open-style model. It is worth considering the approach early in the development cycle, as many of these approaches are intrusive, by syntax or language.