JSR-310JSR-310 is based on nanoseconds, not milliseconds. As such, the minimal set of sensible methods are based on hour, minutes, second and nanosecond. The decision to have a nanosecond base was one of the original decisions of the project, and one that I strongly believe to be correct.
Adding a method for millis would overlap that of nanosecond is a non-obvious way. Users would have to think about whether the nano field was nano-of-second or nano-of-milli for example. Adding a confusing additional method is not desirable, so the method was omitted. As pointed out, the alternative get(MILLI_OF_SECOND) is available.
FWIW, I would oppose adding the getMillis() method in the future.