Is marketing worth it if my clients come from referrals?

It is worth it the moment you want to grow faster than word of mouth carries you. Referrals are the best traffic you'll ever get, but you can't turn them up on demand - marketing is the only channel where you decide the spend and can expect a result in return.

If you're fully booked and don't want to grow, you only need enough marketing to keep your name visible and be findable when someone looks you up. That's cheap and a minimum will do. The moment you have spare capacity, every week without marketing is a week you pay rent and wages out of smaller revenue than you could have had.