I'm not clear on what the conceptual hurdle is. Of course it's a common pattern--that doesn't mean it's always necessary. It does make a lot of things easier,

I'm not clear on what the conceptual hurdle is. Of course it's a common pattern--that doesn't mean it's always necessary. It does make a lot of things easier, like testing, etc. It's the same thing as declaring your vars as `List` or `Collection` instead of `ArrayList`. If you do the same thing in your system you give yourself much more flexibility.