To give some context on the trade reference I referred to, it was an internal tracking within the STP system used solely for tracking state and for communication between IT and the business. It was a simple date + numeric value used in our STP system for users to use in our Struts web and C# front end to check trade state through the STP flow and communicate with us if issues arose.
We did have lots of interfaces where it made sense and relied highly on object composition to represent financial concepts more richly and for inject-ability via Spring and Unit testing (makes writing tests easier when you mock things out). Asset types, security identifiers, etc, were represented correctly from an OO perspective and were a part of the xsd layer/interface between us and the trading systems. To us these were read-only values we just passed through for STP.
You are right about interfaces and unit testing but this is one case I highlighted of many where I think the lead was going over board. The internal trade reference was the same for 8 years and still the same to this day (which gives it another 3 years since I left for a total of 11 years). It never had more than one concrete class. It is no big deal on its own, but when combined with the other interfaces that only have one concrete class, it just bloats the system for no good reason.
Design and architecture is good for the reasons you have mentioned and more but it can go over board as is the case here IMO. There were other instances of that in our code base but that would take an entire blog post to cover some of the atrocities this engineer created because of his forecasting ability.