It's not just the mental ease, it's also the physical typing ease (and in some cases, the possibility).

For example, he points out that to connect C to existing parts of the system (which is the OS and OS level tools), all you have to do is call the functions. If you want to call a C library from a Java program, it's a lot more work. Furthermore, C has the capability of understanding Java structures (although it's awkward), but Java has no way of understanding C structures from within the language. There is no way to model a driver I/O port in Java, but in C there is.

The paper is worth thinking about. If you are creating a language, take interoperability between already existing languages into consideration. JNI is ok, but think how much better it could be if it did auto-marshalling of objects!