A significant part of the lines of code in any enterprise Java application are devoted to plain old java objects (pojos) that carry data. Although they are simple, they represent a non-trivial effort in development and testing. And they make an application resistant to change because often a small change requires alteration of code in several places. Pojos written in Scala are so simple that the value of mixing Scala into Java development becomes worth the extra startup effort.
I've always been one of those perhaps slightly annoying people who don't easily accept propositions that seem overly intricate. It's not that I'm a stroppy person - I'm pretty relaxed really - I just don't like wastefulness, and that can mean I'm unhappy when I see other people wasting their time doing things the hard way. I suppose I was trained this way by the years I spent working with the Occam programming language, a language that espouses explicitly the avoidance of complexity by making behavioural semantics really clear to the developer.