The main concern I had was that both the maven-compiler-plugin and the scala-maven-plugin were compiling Java code. So we were compiling some classes twice for no reason. Furthermore, scalac is slower than javac, so I wanted to cut down on the amount of work it has to do for efficiency reasons. 

I have more trust in the maven-compiler-plugin handling Java code.  Furthermore, not sending the Java classes to scalac means we can't use Scala code within the project in Java code within the project, but we have not run into a need to do this in other mixed Java/Scala projects we've created. Plus this can be easily reconfigured should the need arise. 

Based on these reasons, my argument is that maven-compiler-plugin should compile Java code, scala-maven-plugin should compile Scala code, and if we ever run into a situation where we want to use in-project Scala code from in-project Java code, we can change this configuration at that time.