tl;dr: skipping out on cataloging won’t significantly impact your ability to be a great librarian.
Like many questions about libraries: it depends. Depends on your future plans, depends on the type of library you work in, depends on what you already know, depends on what the class covers. For example, if the class only covers Dewey and LC, you can learn the basics of those systems on your own and unless you plan on becoming a full time cataloger, that’s all you need to know. IMO, you would benefit far more from a class on metadata and the intricate details of database systems (e.g. how different search algorithms work).
The future of libraries will not be found in the organization of information: we’ve moved beyond that possibility (cf. David Weinberger’s book, Too Big To Know). It’s now our job to help people find it.
PS- Cataloging is my primary job responsibility.