Since the release of HubCMS, the www subdomain is not available as a domain option, without the CMS licence. Previously it was available to host your Marketing hub landing pages, which you could set up as a mini website.
I can see a sweet spot for those clients transistioning from other CMS’ to HubCMS. We have starter for all other Hubs, why not CMS. It could offer a limited feature set, ie: only 1 active A/B test, limited SEO and no blogging.
These are all features that are an easy up sell for partners, once you have a client on the platform and seeing results.