Is catclust.sql really required? What is the importance
Let me start with another question.

Is catproc.sql/catalog.sql really required?

Try creating a new database manually:

create database;

And stop here, don't run any scripts. You have a "functioning" database, you can create tablespaces, users, query v$ views, create tables, indexes, insert and query data. You can even create user-managed backups.

Now try to run PL/SQL code, query ALL|DBA|USER_* views, start RMAN, use AWR or jobs. They will not work, because the objects they need were not created.

Can we use the database without running the above scripts? Yes we can, but some feature may not work, and life becomes quite uncomfortable. For example, the view DBA_DATA_FILES is created by the above script, but that view is not mandatory for the operation of the database.

Can we use RAC without running catclust.sql? Yes we can, but some feature may not work.
We can view the content of catclust.sql, but to be honest, I don't know what features depend on the objects created in there, so I just follow the documentation, and run the script.


  Why isn't the component showing up as installed?


Because that is nothing more, than an extra row in a specific table (SYS.REGISTRY$), we can insert anything in there. And the required script was not run.


  And, if the documentation states that catclust.sql create rac
  additional views, shouldn't it be a mandatory component for RAC dbs?


No, as it (in my opinion, based on the content of the script) does not add any additional core feature, just extends the existing ones.