The index is a power user feature. Its forced presence in Git effectively constitutes a usability barrier for new users. After all, a VCS is effectively a glorified abstraction for "save a file." Any barrier imposed between changing a file and committing it can get in the way and confuse people. The Git index does this.

Furthermore, the index is effectively a pseudo commit without a commit message. Any workflow using the index can be implemented in terms of actual commits itself.

I think because Git doesn't have strong usability in general and especially around history rewriting, many Git users feel that the index or an index equivalent is somehow a required feature of a VCS because Git's shortcomings give that illusion. However, if you use a VCS with better history rewriting (such as Mercurial with evolve), you'll likely come around to my opinion that the index can be jettisoned without meaningful loss of functionality or productivity.