Ghost currently does not include a built-in table of contents (ToC) - it should.
Why have a ToC in a post?
In this age of TL:DR, with anything beyond a trivial post, you need to give the reader some clues about the juicy goodies further down your post, and a means for them to jump ahead if they wish.
Why not DIY or use the Ghost Tutorial version ?
To produce a sticky, responsive ToC is a challenge for many users. Since any post constructed and served through a Ghost server is a PR piece for Ghost, if the home-hacked ToC looks bad then Ghost looks bad. By the way that Ghost tutorial version is not responsive.
Additionally
On the plus side: Such a ToC would be a differentiation for the then builders, meaning any theme with a sticky responsive ToC would be more attractive to purchase than those that do not contain this feature. Also, Ghost Pro base theme could include a ToC as an extra encouragement.
On the negative side: Ghost’s marketing talks about
Fiercely independent, professional publishing
The product marketing manager might like to consider that without something as standard as a ToC, those claims are rather transparent.