Yes. Paying an expert in legal SEO is worth it for lawyers who want to grow their client count through organic search.
Most lawyers do not have the time to handle all the small details of an SEO campaign. And that's before adding the effort it would take them to teach themselves everything they need to execute on their own.
Lawyers are better off working with a partner who focuses on legal SEO than trying the DIY route. Your law firm SEO partner will be able to optimize your site, create relevant content, and handle the technical details for you. That leaves you to handle the influx of new clients to your firm.