In the ProcessPayment example he says the pattern is good because it lets you split the construction of the payment from its execution.
Soooo why not have a Payment class with a process method? Am I missing something?
It dawn on me that the latter is precisely what people at Java do, and I've reached the conclusion that this way of writing software is enforced by Java developers who realized they needed to learn Ruby because their job prospects are better, and are simply retrofitting what they used to do in a new programming language.
Extracting important business behaviour into objects allows applying OO (ie, powerful despatch rules) where it is valuable.
Subtyping your model to achieve many of these possible usecases would give poor design results.
For example, if payment processing were in Payment.process() the Payment entity would need to be subtyped or composed for any of 1) a different payment gateway, 2) a sales tax in a new jurisdiction, 3) a new confirmation, or 4 a new payment flow. Having all that in your model entity is probably wrong.
Simple behaviour is fine in the model, major business TN usually not.
There's nothing at all wrong with having, say, a PaymentGateway and SalesTax classes and composing them with Payment in whatever way makes sense given their interdependent relationships. Code that has real life analogs to objects tends to be much cleaner, in fact.