Not really: using modules is completely optional. Small applications likely won't benefit from the structure and guarantees modules offer. Large-scale applications, however, will certainly benefit from strong encapsulation and explicit dependencies all enforced from compile-time through run-time (and adding an optional linking step for modules with jlink).

(full disclosure: I'm author of Java 9 Modularity, O'Reilly, see https://javamodularity.com)