This only makes sense if you work with different languages and/or a plugin architecture. I know that what you say is written in the books but in practice in 95%

This only makes sense if you work with different languages and/or a plugin architecture. I know that what you say is written in the books but in practice in 95% of the cases you will never write your new ser ice in language X and you will never switch from Hibernate to EclipseLink, etc. I'm not saying that abstractions are not useful. What I'm saying that over-architecting something on the possibly false premise that "at some time we might need to support X so let's do it by creating an additional abstraction" will waste time, money and will just end up as code clutter which you have to maintain.